. . . and then the Doctor felt himself yanked forward so quickly he was barely able to keep upright.

The robot had been playing with them, judging their strength. Now they were running forward, unable to stop. The first athlete reached the red line almost before they knew what was happening.

With a crackle, a wall of energy sprang from the line. The girl barely had time to scream before it hit. When the last of the blue flashes died away, there was nothing on the ground but dust.

‘Don’t let go of the rope!’ yelled the Doctor, as the others stood there, stunned. ‘Keep pulling!’

He knew now that they didn’t stand a chance. He wouldn’t let them give up, though.

‘It’s too strong!’ sobbed the young woman behind him.

‘Yes, it is,’ said the Doctor.

But something wasn’t quite right. The robot was strong – but not as strong as he had expected. ‘It’s feeling Earth’s gravity!’ he cried. ‘It must have controls to adjust its apparent mass, like the Sontarans’ suits. If they were set for Sontar, it would seem even stronger!’

He peered hard – yes, there was a dial near the robot’s head. The controls!

‘If we could only get close enough, I could change things,’ the Doctor told the athletes. ‘I could make it feel really light, so we could pull it over the line easily.’

‘But we can’t get that close!’ shouted a youth, as they were dragged towards the line again.

‘I know!’ cried the Doctor. ‘What we need—’ He broke off.

He couldn’t believe his eyes. He’d have rubbed them, if he’d been able to let go of the rope. Surely he was dreaming?

Crawling across the arena, towards the robot, was Emma.

Two of the Sontarans were watching the crowd. The other two had their guns pointing at the Doctor’s team. None of them were looking towards the robot. He had to make sure it stayed that way.

‘Tell you what, let’s sing a song!’ he called.

‘What? Are you joking?’ shouted a young man from somewhere behind the Doctor.

‘Not at all! A song to keep us all pulling as one. Just like the work songs from old America. The slaves would sing as they worked, to keep a rhythm. Maybe even to pass coded messages under the noses of their slavers. Come on! I’ll sing the verses, you join in with the chorus! You’ll all know this one!’

He began to sing at the top of his voice:

Swing low, sweet chariot,

Coming for to carry me home.

The rest of the team slowly took up the chorus:

Swing low, sweet chariot,

Coming for to carry me home.

The Doctor began to belt out a verse:

I looked at the robot and what did I see,

Coming for to carry me home.

A dial by its head that turned down should be,

Coming for to carry me home.

He glanced at the Sontarans. They didn’t seem to have picked up on his message to Emma. She gave him a wave and started to climb up the robot’s leg. He joined the team in belting out the chorus again, then added another verse. He hoped Emma was listening closely.

When the dial goes down,

the weight goes down too,

Coming for to carry me home.

Then jump off

or the line will frazzle you,

Coming for to carry me home.

Emma was on the robot’s shoulder as they sang the chorus again.

The Doctor kept singing to his team:

When that’s done everyone must pull together,

Coming for to carry me home.

It’ll feel to us even lighter than a feather,

Coming for to carry me home.

He saw Emma’s hand reach out and grab the dial just below the robot’s head.

She turned it.

The Doctor’s team didn’t pause. They just kept pulling.

‘Stop!’ he cried, but it was too late. For a moment, he thought the robot was flying. It came towards them like a bullet out of a gun.

The robot flew over the line. Energy beams leapt up from the ground. There was a crackling sound, then a huge explosion of blue and green. When it cleared, the robot was gone.

There was no sign of Emma.

The Doctor turned to his team, who were all crying with joy. ‘The girl who was up there,’ he said. ‘Where did she go?’

‘I didn’t see her jump down,’ said one, and the others all shook their heads.

‘I don’t think she had time,’ said another. ‘I think she was still on the robot when it came across the line. Oh no!’

‘You should have let her get off before you started pulling,’ said the Doctor. But he said it very quietly. He knew he hadn’t been clear enough. Hard to let people know what to do in a song. It wasn’t the team’s fault.

His hearts sank. No human could have survived that flash of energy. If Emma hadn’t got off the robot in time, she would have been fried.

Chapter Thirteen

‘DOCTOR, LOOK!’ CALLED Holly’s voice from the stands.

A red laser beam flew over the Doctor’s head, and he turned.

Holly hadn’t been shouting a warning, though. She’d been trying to tell the Doctor about something else.

All four of the Sontarans were floating off the ground. They were trying to aim their guns, but couldn’t manage it. Every few seconds they came back down to earth, but each step forward pushed them up again.

The Doctor gave a huge laugh. ‘All their gravity controls must be connected to a central matrix! When Emma changed the robot’s mass, the controls in the Sontarans’ suits were affected too!’ He shook his head, still grinning. ‘Well, that’s a clone race for you. What’s good enough for one . . .’

Then he stopped smiling. There was no time to waste. This was the chance he’d been waiting for.

He ran to the tunnel that led to the gym. OK, so the building was sealed, but at least they’d be out of the way of the Sontarans.

This door was now locked too. Sealed by the Sontarans.

Back in the arena, he looked around, searching for clues. If only they could disable the Sontarans. But if anyone went near them, the Sontarans did their best to shoot. How long would it be before one hit its mark? Getting close enough to hit a Sontaran’s weak spot, its probic vent, wasn’t possible.

The Doctor looked upwards. The bundle of missiles was still hanging beneath the force dome.

The weapons were held in a force-sphere. Spheres often had a weak spot at the join. The Doctor stared. Yes, there it was. He could see a flicker of energy, a little white hole buzzing near the top. If he could get to it with his sonic screwdriver, he could release the missiles.

He glanced around the arena. Surely he could come up with a plan using a load of top athletes and a long rope . . . Yes!

‘Tug of war team, to me!’ he called.

The six strong young people ran over to the Doctor. ‘Three this side, three that side,’ he told them. ‘Stretch the rope across, then climb up the stands as high as you can. Hold the rope tight.’

They didn’t even ask why. They just nodded and ran off to do as he asked.

‘Holly!’ called the Doctor.

She jogged over to him.

‘How are you feeling? Head OK?’

‘I’m fine,’ she said. ‘What do you want me to do?’

‘Well, I’m led to believe you might be the best gymnast in the country.’ He pointed to the rope, now stretching across the arena at a height of several metres. It passed below the force-sphere containing the weapons. ‘Could you get from that rope to that sphere?’

She nodded. ‘With a little bit of help. Piece of cake!’

‘The energy won’t hurt you,’ he told her. ‘It’s just holding everything in one place.’ He took out his sonic screwdriver, and adjusted the settings. ‘Just push it through that little white hole. I’ve set it for a 10-second delay. That should allow you to get off safely before the sphere vanishes. Do you think you can do that?’