Gordon sat down heavily in the pilot's chair of Thunderbird Four and turned to Tin-Tin. "I lied," he said.

"You… lied?" Her heart was in her mouth. Her first rescue as an official full IR operative, and what she wanted was something easy, straightforward, and safe.

"I told Scott I was fit to dive. I'm not. You'll be going into the wreck. Don't worry – I'll be right here minding the bus."

Three minutes to drop, said Virgil's voice over the intercom.

Tin-Tin glanced at the door. Should she tell Scott? Maybe. But then Scott would put himself on the rescue team. And… Scott wasn't that good a diver, no matter how much he practiced and tried to make sure he was competent in any role IR could need him for. No, she'd rather have Gordon in here, even if he was full of cold. He sounded more congested now than he had even half an hour ago, when he'd stood in the lounge of the villa and laughed off suggestions that maybe he should sit this one out.

She trusted Gordon. If he said she was ready, she was ready. And the wreck of the Hesperia wasn't that deep. A good technical training dive, Gordon had told her. If he hadn't returned from his week off with a streaming cold, they'd have been diving it themselves yesterday, as the next stage of her introduction to penetration diving.

Drop in five.

She held onto the edges of the jump seat – Brains had talked about upgrading it, but other, more important things had come up – and breathed out as the bottom fell out of the world and Pod Four splashed into the Pacific.

Gordon, I have the dive boat on the line.

"Put them through," he said, clearing his throat. "Dive boat, this is Thunderbird Four. We'll be going in to look for your missing people – what can you tell us about them?"

Tin-Tin sat and listened, trying to remember the important information without getting affected by the emotions of the woman on the other end of the radio. It was her husband and fourteen year old son down there. The boy was relatively experienced for his age. The father had been a sport diver for years, this wreck should have been well within his abilities, but he had never done any more difficult or technical diving. And the mother had been a diver once, but ear problems had forced her to give it up and now she manned the dive boat when the others went out. She knew something was wrong. Badly wrong, despite her professional calm.

"Bill never goes into his reserve. Ever. He should have been back an hour ago,"she finished. No hysteria. No begging for help. She knew who they were, and what they did, and that there might not be anything that they could do.

"We'll do our best, ma'am. Keep this line open." Gordon turned the transmitter off, sneezed hard, and blew his nose.

"You check your gear and get into your suit. I'm going to run a wider scan as we descend. No point going out there if they're decompressing fifteen feet down."

She knew they wouldn't be, though. An experienced everything-by-the-book diver like Bill would have sent a flag up to the surface. No, they were in the wreck, on their planned dive, and probably one of them had got hurt or entangled. They should have half an hour of air left. All she had to do was to follow the guideline and she'd be led right to them.

By the time she'd climbed into her wetsuit and checked her own breathing apparatus, Gordon was laying out the rest of the kit. Cutting gear, mostly. A jack to force apart anything which might have fallen and trapped someone. A spare air bottle. Spare masks. A reel of guideline. A main light, and two backups. Multiple emergency flares. She couldn't possibly have carried it in air, but she didn't need to. She'd attach it to the diver propulsion vehicle while sitting on the floor of the airlock. The water would take the weight, and the DPV would be doing all the work.

"Tell me about the route," he said.

"They're on the red route. There's a permanent fixed guideline, but he'll have set up their own secondary. Their guideline cord is blue and yellow."

Gordon nodded. "I'm not going to tell you how to dive. Just be careful, Tin-Tin. I didn't want you doing this on your own, not first time out, but…" With perfect timing, he sneezed again.

"I will be fine." Mostly she was nervous of dealing with two panicking divers, but she knew the chances of that were slim. If they'd panicked, it would have happened already.

She glanced out of the front window. Gordon had extended the grabs to secure Thunderbird Four to the wreck, and turned on the floodlights. She could see rails, hull, metalwork, and, over to the right, the red guideline reel, a large red marker, and a dark hole in the side of the wreck which must be the entrance to the red route. There was no sign of a blue and yellow line.

"She did say he'd use a secondary?"

"She did. I don't see it." Gordon blew his nose again – they'd pressurised to the wreck depth, and it couldn't be helping his congestion any. "Maybe he decided not to, since there's a fixed line. Or maybe it's fastened inside. You will use yours."

"Yes," she said, since he seemed to need the reassurance. "I'm ready to go."

Gordon picked up the equipment with an effort, shifted it to the airlock, and the two of them attached it to her, piece by piece, making sure nothing could fall off, or become tangled, or damage anything else. You didn't rush when diving, he'd taught her. You never rushed. Everything had to be right first time, so you double-checked that you had everything, that your air supply was full and working properly, that all your lights had working batteries, that you had the emergency stop line correctly attached to the DPV, and so on.

"Set," she said finally.

"Good luck, and take care," Gordon said, heading back to his seat. "And… Tin-Tin? If there's nothing you can do, get out of there. Don't put yourself at risk."

"I won't." She knew that had to be a possibility – that she'd find two bodies. But there was a strong chance that they were still alive. Normally International Rescue wouldn't attempt this sort of rescue simply because diving accidents had to be resolved so quickly. The Hesperia, though, was barely sixty miles from Tracy Island. It was still less than an hour since the call had come through. Surely they would still be alive, just in need of help? A broken mask, maybe? Possibly even some sort of large sea creature cornering them? Her kit included various things to stun or scare off sharks or giant squid, though she'd never used them.

The water was warm and pleasant. If there hadn't been people in danger down here, this would have been fun. Tin-Tin engaged the DPV and directed it towards the entrance to the wreck. There was no blue-yellow line here. She made quite sure, checking every possible anchor point inside and outside the opening before fastening her own yellow line alongside the red line.

Progress along the corridor was quick and easy. Still, she did everything by the book, being careful not to stir up any silt, checking her gauges (not that she was exerting herself; the DPV did all the work for her) and making sure that every door she passed was solidly shut. This part of the wreck had been deliberately made safe for relatively inexperienced divers, Gordon had told her – little or no chance of getting lost. They'd planned to come in here first before heading round to the other end of the ship so she could get some experience in less friendly surroundings. She desperately hoped that Bill hadn't decided to do the same. If so, she was very unlikely to find them. But it wasn't in his character. At all. She held onto that hope.

At the end of the corridor, the red line snaked away into the darkness across a relatively big open area. This was the first place they might be, if something had gone horribly wrong. Tin-Tin attached her personal line to the main guideline before checking systematically, one corner after another. It wasn't difficult. The water was clear, the furniture had been removed, and even the far corners were only barely out of sight of the guideline with the powerful light of her DPV. They weren't in here, and there were no clues – no abandoned equipment, and still no sign of that blue and yellow line she'd been expecting to find.

She detached her personal line and moved on, across the room and into the smaller corridor at the other side, following the red line. She knew it wasn't much further until the route dead-ended. Please let them be down here, she wished desperately. Cold, hurt, scared… just let them be alive. She so didn't want her first solo rescue to involve towing bodies home.

Again, every door was sealed shut, and there was no chance of missing anyone in this corridor. It was barely ten yards long, and then it opened out again. This had been the main lounge area of the ferry, she knew. It was significantly larger than the other room.

The red guideline terminated at the entrance to the lounge. Tin-Tin attached her personal guideline to the metal ring, fumbling with the connection. It felt much colder than she'd expected in here, in tropical seas in the middle of summer.

The lounge seemed huge, as she set off along the wall. She'd seen the dimensions – could it really be taking her this long to get to the first corner? Was the DPV failing? She'd never be able to extract two struggling divers without it.

Don't you dare panic! She refused to allow her breathing to speed up. Long hours of training kicked in, and finally there it was: the first corner. Empty.

Three more chances, she told herself, and kept going. Assuming they were in a corner, of course. If not, she'd have to spiral into the centre of the room and check every pillar, every table. The visibility was dreadful in here, and getting worse, surely. Which made no sense. There was almost no silt, and she'd been so careful not to stir anything up.

Sixty feet to the next corner, so why did the DPV readout say she'd already gone eighty? Tin-Tin blinked hard, and it became fifty. Narcosis? Just confusion? It didn't matter. She should turn round and go back, right now. If she did, two divers would die. She kept going, and there they were, huddled together in the junction between walls and floor. Both of them together. Two streams of bubbles. The relief was so intense that she had to blink back tears.

The diver on the right responded sluggishly to the light and her hand on his arm. The other one, smaller, must be Jeremy, the son. He appeared to be unconscious, or at least unresponsive – but he was breathing. Tin-Tin extended another short line and was clipping it to his belt when Bill seemed to come to life.

Danger, he signed. And go back and low on air.

She should have checked that first. But she had a solution for it. She unhooked the emergency supply from the DPV and passed it to him, checking the gauge as she did so.

Less than half full. But it had been completely full. She knew it had. She'd checked it. Gordon had checked it. This time, blinking didn't help.

Her own supply was fine. Not that she had any confidence in it any more.

Bill appeared unable to do anything requiring fine motor control, which maybe wasn't surprising given the cold. He'd been down here for a couple of hours now. She set up the additional air supply for him, and then for Jeremy. The kid's air supply was close to exhausted – she'd got here just in time. They needed to go.

She clipped Bill to the DPV too – she didn't like having all these potential tangles, but she didn't feel there was a choice. Visibility was so bad that if Bill let go of the DPV and drifted even a few feet away, she'd struggle to find him again. Where had all this silt come from?

How she wished Gordon was here.

And then the DPV's headlight flickered and went out.

Don't panic! She knew exactly where the emergency lights were. Not as bright as the DPV light, or as bright as she'd have liked – or as bright as she remembered. But it was plenty bright enough for her to follow the two lines back across the lounge; her yellow one and their blue-yellow one, which snaked off side by side into the darkness. Why hadn't she seen theirs before? She should have checked around the door more carefully when she'd been attaching her own. No matter. She had it now, and all she had to do was follow them out.

She checked Jeremy's regulator was firmly in his mouth, shortened the line to keep him as close to her as possible, and put Bill's hand on the secondary DPV towing point, making sure he had a good grip on it.

Ready? she signed.

Ready. Bill put his free arm around Jeremy, and Tin-Tin set the DPV moving again, reeling in both lines as they went.

It always seems further in bad visibility, Tin-Tin told herself. And breathe evenly and reassure your rescuees. The first two were from Gordon; the third from Scott. Of course, all Scott had to do to reassure someone was to smile at them and say something in that deep melting voice of his. It didn't work quite like that underwater.

Finally, the wall, the entrance to the corridor, and the fastening point. Her personal line attached next to the blue-yellow line.

No red line.

No continuation of her line, either.

No marker for the way out. Just a gaping hole into blackness.

She glanced sideways. Bill's face was white and terrified behind the mask, his jaw clenched on the regulator. She was lucky he wasn't in hysterical panic, she realised.

This had to be the way out. This was an easy, sanitised training dive, without alternative exits providing ways people could get it wrong. Somehow the red guideline had broken. And her main line. Or could she have unfastened the red line when she thought she was attaching her own? And her own main line? No, surely not – fixed lines were attached in a far more permanent way than individual lines. And the blue-yellow line had not been here. It just hadn't.

There was no alternative. She set off slowly into the dark.

They'd gone barely six feet when Bill grabbed her left arm. His eyes were wild and desperate.

No!

Back!

No!

He signed over and over again, pulling at her, spinning the DPV round, stirring up the silt even worse. She couldn't see. She didn't know which way was up, or down, or out. She knew she should push away, let him panic himself to exhaustion… but that would mean letting go of the DPV, and she couldn't do it.

There was something down here.

Something black, and malevolent, and it wanted them scared and lost and dead.

And it was coming.

Oh, Gordon! Please realise something's wrong! I can't do this. I can't get us out. I need help, right now.

A light glimmered through the murk. Just faint, but it was all she had. Tin-Tin managed to detach Bill's desperate grasp from her arm and pointed, right in front of his face to be sure to get his attention.

That way!

He was gasping uncontrollably and she didn't want to think about how much air he'd have used in the last minute or so, but he did respond, grabbing the DPV again. Tin-Tin threw caution to the wind and hit the speed on the DPV. Visibility couldn't get worse, and she had to get away from the creeping horror in the dark.

Back into the lounge, and there was the source of the light: a lone diver wearing the insignia of WASP rescue. He didn't wait for them to get within touching distance, signing follow and heading away from them. Tin-Tin needed no second invitation.

He turned into a side passage without slowing, and she followed him blindly, desperately, knowing her own breathing was far too fast and too terrified to do anything about it. Beside her, Bill's face was white. The gauge on the bottle that he and Jeremy were sharing read near-empty. Tin-Tin considered stopping to share her own supply, but she didn't dare. She couldn't lose sight of the man they were following, who was as fast as the DPV at top speed when it was towing three people. Gordon had, of course, been an Olympic champion. He'd also been a WASP rescue diver, and had far more sense than to come within grabbing range of a man who'd already panicked once.

The exit hatch. Thunderbird Four just outside, lights blazing. And the start of the red route, complete with the permanent red line, and alongside it, her yellow line and Bill's blue-yellow one.

She throttled right back on the DPV, letting them drift towards the anchor point. Her yellow line reeled itself in neatly and just needed to be detached. The blue-yellow was tight and vibrating, as if something was pulling on it. She left it alone.

When she looked up again, Gordon was gone. Four people wouldn't fit in Thunderbird Four's airlock at once. Even three was going to be tight, but she wasn't going to leave anyone out here alone. The DPV wouldn't fit, but there was an external locker next to the airlock door for situations just like this. The airlock door opened, and she unclipped Bill and guided him inside before detaching Jeremy and passing him to his father. Still unconscious. Still breathing. Finally, she unclipped herself, pushed the DPV into the locker, sealed the hatch, wriggled her way into the remaining space in the airlock, and pulled the door shut behind her. No way to get a view of the airlock controls, but she didn't need to see them to start the pumping-out sequence.

She'd got everyone out. Well, Gordon had. For now, that was all that mattered. She had no doubt that, later, he'd be re-evaluating his decision to allow her to be the second active member of International Rescue's underwater team.

The last inches of water drained from the airlock. Tin-Tin was just considering how to reach the door release, currently behind both Bill and Jeremy, when the inner door opened. Gordon caught Jeremy as he crumpled unsupported into the main cabin and lowered him to the floor.

He started medical checks immediately, but Tin-Tin knew what he was going to need. She slipped past him and pulled self-heating blankets from the locker under the console even as Gordon reached round for them.

Bill was sitting on the airlock floor, weeping silently and wretchedly. Tin-Tin put a blanket round his shoulders, and went to help Gordon. First aid on a fully suited diver was awkward to say the least. Someone else to remove cylinders and weights made things much easier.

Jeremy was stripped of equipment, peeled out of the top half of his wetsuit, and wrapped in heated blankets, and Gordon continued to watch him closely. He'd made no move to start decompression – they'd worry about that later, possibly after they'd discussed it with experts.

"Where's the other diver?" Bill said suddenly.

"Other diver?" asked Gordon.

"The man who came in after us."

Gordon frowned, and something twisted in Tin-Tin's chest. Gordon's hair was bone dry. He was clad in his IR uniform. No wetsuit on the floor. Not that he wore a WASP wetsuit any more – she'd seen pictures of him in it, but his current suit was black. And in any case there was no way he'd had time to change.

He hadn't been out there.

"You're kidding me," said Bill. "He's not with you? Where did he go? I can't thank you two enough for what you did for us… but I want to thank him too."

"Tin-Tin?" said Gordon.

"There was a WASP rescue diver out there," she said. She wanted to add I thought it was you, but Bill was no idiot and she didn't want to tell him that International Rescue's diving expert had been a WASP rescue diver. It was too small a world.

Gordon stood up stiffly, leaving Tin-Tin to watch Jeremy, and crossed to the console. Checked something. Checked it again. Frowned.

"Did he come out?"

"Yes," she said.

"Just ahead of us," Bill said. "Didn't you see him? Real athletic guy. One of those WASP wetsuits with the red flashes that their elite rescue divers wear. Mismatched tanks. Oh, and missing two fingers on his right hand."

She doubted Bill would have picked it up, but Gordon went completely rigid, his hands locked on the controls. "Oh, that diver," he said unconvincingly. "He's fine."

"How do you –"

There was a splutter from Jeremy. His eyes flickered open, and he struggled to sit up. "Where is it? It's coming!"

"Easy, son," said Gordon reassuringly. "You're safe now."

And the teen's eyes widened in recognition. "International Rescue? You… I… Dad!"

"It's fine, Jem," said Bill. "It's all going to be fine." And he put an arm round his son, wiping his other hand across his eyes.

"Let's get you home," Gordon said, sitting down in the pilot's seat, and Thunderbird Four began to move. Which was fine by her. She never wanted to see the Hesperia again.

She'd been worried that she'd have to confess her failures in the debrief, but in the event it didn't happen. By the time they'd decompressed, Gordon was alternately sneezing and blowing his nose, clearly very uncomfortable, and Scott rescheduled debriefing for the next day. He had not looked impressed.

"Ten minutes," Gordon muttered to her as Scott stalked ahead. "My room."

No, she'd have to explain herself in private. Which wasn't a whole lot better.

When she got to his room, Gordon sounded at least somewhat better, probably due to the bottle of decongestant sitting on his desk.

"Tin-Tin, do you believe in ghosts?"

That wasn't what she'd expected. Not from the ultra-practical Gordon. Not from any of the Tracys. She wasn't sure how to answer it. Ghosts, in the Western tradition? No. Spirits, presences, things which science couldn't explain? Absolutely.

She said nothing, and Gordon held a picture out to her.

"Is this who you saw?"

Two men in diving gear sitting side by side on a boat and smiling for the camera. Gordon, a few years younger than he was now, and an older man. Both wore blue wetsuits with red flashes. The older man had mismatched cylinders, and two fingers missing on his right hand.

"Yes," she said. She hadn't been thinking clearly at the time, but she'd seen the missing fingers in his hand signals. And yes, those cylinders, with the larger, less battered one on the right.

"That's Ted Hawkins," Gordon said. "He was training to be a WASP rescue diver with me – he was older, he'd been a recreational diver and wanted a career change. Good diver. Lovely man. Looked out for everyone. He had a stroke at three hundred feet on a training dive. We broke every deco rule we dared to get him to the surface, but… The doctors said he would have died instantly. Damn shame."

"I'm sorry," she said.

"If it's possible to come back, this is exactly what he'd have done. Sometimes all it takes is a visual reference to get a diver out of trouble when they're disoriented."

"Yes," she said. "It was him. But... Gordon, it wasn't just him. There was someone…something…else down there. The safety lines moved. They weren't there, and then they were, and then they were somewhere else."

He looked at her, considering. Her future with International Rescue was in his hands.

"I'm sure."

He nodded, but she didn't think he was convinced.

"Go get some rest," he said. "We'll discuss this again tomorrow."

It didn't last that long. Half an hour later there was a knock on the door of her room, and it was Gordon.

"I took a look at the DPV's video," he said. "I'm sorry I brushed you off, Tin-Tin."

He held out two more pictures.

One had a timestamp of a couple of minutes after she'd left Thunderbird Four. It showed her hands attaching her yellow line to the anchor point next to the red line.

The second had a timestamp around twenty minutes later, and showed her detaching the line again. Next to it were a red line and a blue-yellow one.

"I watched the screens the whole time," Gordon said. "Nobody else went in or out, or anywhere near. So when I saw that, I looked up the history of the Hesperia. She went down in a storm. One death: a multiple murderer who was locked up below decks."

He gave her a third picture. This was a classic police mugshot – full face, holding a number board in front of a set of height lines. He had a hard, sneering expression and cold eyes, and she saw again a dark corridor which should have been sealed, leading to the bowels of the ship, where a prisoner might well have been transported.

"The two men responsible for him each swore that they thought the other had made sure he was evacuated," Gordon said. "He'd killed five children. I'm not sure I'd have bothered either."

She looked up from the picture, trying not to relive the feelings of cold hatred it had reawakened in her. "What do we do now? What if it – he – does it again?"

"I don't think he will," Gordon said. "The Hesperia sank one hundred years ago today. That's a bit much of a coincidence. But just in case, I'll have a quiet word with the WASP chaplain. From things I've heard, it won't be the first time he's dealt with something like this. Though I'm not quite sure how bell, book and candle work underwater."

She laughed, shakily.

"That's better," he said. "But…for the debrief? I think there was a big octopus down there, or maybe a shark. Had Bill and Jeremy cornered. The bright lights and the DPV's emergency sonics scared it off. Textbook rescue."

The last cold knot of fear in her chest untied itself. Bill and Jeremy were safe, Gordon knew what had happened, and nobody else needed to.

And, should anything like it ever happen again… maybe Gordon's friend would be there to help.