‘Ah, but you didn’t know what you were looking for!’ he said.

Karl looked hopeful. ‘And you do?’ he said eagerly.

‘Well, no,’ said the Doctor, and the students sighed. ‘But I am an ace private eye with a degree in detecting and a licence to sleuth. I’ll work it out.’

They reached the pool room. The Doctor asked Karl to stay on guard, watching for Sontarans. The sprinter seemed relieved not to have to come inside with the others.

The Doctor flicked a switch as they entered, and the pool area was flooded with harsh white light. Out of the corner of his eye he saw Emma shiver.

‘Cold?’ he said.

She shook her head and pointed at the pool. ‘That’s where she was,’ she said. ‘Laura. You could just see her hair, floating out on the surface like seaweed.’

The Doctor patted her shoulder awkwardly. ‘Don’t worry. I’m not going to let anything like that happen again.’ He paused for a second, trying to think of something else to say. ‘You know, it is a bit chilly in here. Indoor pools are usually toasty warm.’

‘The heating went out with the first power cut,’ said Emma, still staring at the water.

‘Ah.’ There didn’t seem to be a lot else to say. The Doctor was itching to begin his search. He started to roam around the room, looking for anything that seemed out of place. Notices on the walls warned about the dangers of running on the wet tiles. The Doctor was still wearing the rubber boots, though, and he was as sure-footed as a cat.

After a few minutes, Sid called over, ‘Is it just me, or can anyone else smell something?’

They all sniffed. ‘It’s just chlorine,’ said Emma. ‘They put loads of it in the pool to keep it clean. You’d get used to it if you were down here all the time.’

Sid shook his head. ‘No, there’s something else.’

The Doctor joined Sid. He shut his eyes, letting his keen Time Lord sense of smell take over. ‘You’re right, there is something,’ he said, and began to sniff like a bloodhound, with his eyes still closed. Only Sid’s quick reactions stopped the Doctor walking straight into the swimming pool.

After a short detour, he was back on the trail again. ‘It’s over here,’ he said at last, opening his eyes. He was standing by a grille in the floor. He peered through it, but couldn’t see anything.

After a few waves of the sonic screwdriver, the Doctor was able to pull up the grating. Using his torch, he examined the opening, and finally reached an arm down the hole.

Emma, Sid and Holly watched with bated breath as the Doctor pulled his arm out again. His hand came into view. It was holding on to another hand, gripping it as if pulling someone up through the floor.

But there was no person on the other end of the hand. There wasn’t even an arm.

Holly turned round and was sick. Sid and Emma looked as though they were thinking of joining her.

The Doctor laid the hand on the tiled floor. ‘Someone’s tipped about a gallon of chlorine over it. They probably hoped to hide the rotting smell. That makes it harder to judge how long it’s been down there, but I’d say about a week.’

‘But that’s before Laura died!’ said Sid.

‘Oh yes.’ The Doctor nodded. ‘And none of the bodies you showed me had a hand missing. There weren’t just those three deaths. There were four.’ He turned and stared at the students. ‘At least.’

Emma opened her mouth, but the Doctor never heard what she had to say. An alarm blared out, making them all jump. Then a hoarse Sontaran voice boomed out of a speaker. ‘Alert! Alert! Humans are still loose in the building! They must be found!’

‘What’s happening?’ cried Holly.

‘Well,’ said the Doctor, ‘I reckon the Sontaran that Sid knocked out has just woken up. The building is sealed, so they know we must still be around somewhere.’

‘We’ve got to get out of here!’ Emma called, running towards the door. She flung it open, but was met by Karl, coming the other way.

‘Sontarans!’ he yelled. ‘They’re nearly here!’

The five looked around in dismay. There was only one other exit, a tunnel leading to the changing rooms. They dashed towards it.

Halfway across the floor, Holly slipped on the wet tiles and landed on her back. Unable to stop in time, Sid fell over the prone gymnast. Karl held out a hand, but Sid yelled in pain as he tried to get up. He fell back, clutching his ankle. Emma tried to help Holly, but she just lay there, muttering about being dizzy.

Heavy footsteps were getting nearer and nearer. A Sontaran shadow fell across the doorway. There was no time now to get to the tunnel, even if Sid and Holly could walk.

Quick as a flash, Emma jumped into the pool, her dive barely creating a single ripple. The Doctor hardly had a second to reflect on her skill, before two helmeted Sontarans entered. He recognised Captain Skeed by the military symbols on his collar, and assumed the other Sontaran must be the one they’d met by the TARDIS. Both held weapons.

‘Ah!’ cried Skeed. ‘Here they are. Did you think you could escape the Sontarans for long?’

‘Not really,’ said the Doctor, joining them. ‘Oh well. We’d better get going, then.’ He began to walk out of the door.

‘Not so fast, human!’ Skeed put up a hand to stop him. ‘Our great leader Stenx has said you must be punished for your conduct. You are to take part in the first ever Sontaran Games!’

‘Really?’ said the Doctor. ‘Lucky old me! Well, I don’t want to miss that.’ He started walking again.

Skeed raised his weapon, pointing it at the three students. ‘These others will also take part in the Games.’

The Doctor waved a hand. ‘What, them? They wouldn’t be much good. One’s got a busted ankle and one’s just whacked her head on the floor.’ He waggled a finger round his ear. ‘Can’t think straight. No use at all.’

‘We will find a use for them,’ said the second Sontaran, starting towards the little group.

‘No, no, all right, they’re coming,’ said the Doctor hurriedly, as Holly and Sid limped towards the door, helped by Karl. He didn’t want the Sontarans getting any closer to the pool. If Emma could remain free, they might still have a chance. But how much longer could she stay under the water? If they didn’t get out of here soon, she would have to come up for air and all would be lost.

The Doctor gave a sigh of relief as the three students finally made it across the floor, and he started hustling them out through the door. ‘Come on, come on, we don’t want to keep the Sontarans waiting. It’s the first ever Sontaran Games, you know.’

He risked a quick glance back. Was that a dark shape at the bottom of the pool? With a sigh, the Doctor turned away as Skeed slammed the door. Emma had been under the water for almost three minutes – could she possibly have held her breath for that long?

The Doctor, Karl, Sid and Holly were led back to the gym hall. Holly seemed dazed still, and was holding on to the Doctor’s arm. Sid leant on Karl, and was wincing in pain with every step.

Skeed seemed interested in the students, and asked the Doctor about them. ‘The damaged one, what is his sport?’ the Sontaran said.

The Doctor glanced back at Sid, unsure if telling the truth would get the lad into trouble. ‘Oh, throwing things mainly,’ he said lightly.

Skeed nodded. ‘Ah. Then he is of little use to us. All missiles have been removed. Is that not right, Lieutenant Slorr?’

The other Sontaran stood up straight. ‘Following my disgrace, I offered Major Stenx my weapon!’ he said hotly. ‘He refused to accept it, as all are needed to find—’

‘Silence!’ Captain Skeed’s eyes burned through the slots in his helmet as he shouted. ‘Do not add to your folly by speaking of Sontaran concerns in front of these humans!’

The Doctor smiled to himself. Dissent in the ranks was always good. Besides, he now knew something he hadn’t known before. The Sontarans were searching for something. The question was – what?