Unfortunately, it would have to wait. The Mistress had run off after disarming him and knocking him to the ground, instead of killing him as he’d half-expected. Clutching his sword tightly, he jogged down the street after her. As he rounded the corner she’d disappeared down, his heart sank when he saw her enter his TARDIS.

So that’s what she wanted: a way off this planet so that she could find an easier one to conquer, leaving the Doctor stranded here and unable to stop her. Putting on a burst of speed, he managed to reach the TARDIS before it dematerialised.

When he entered the blue box, she looked surprised to see him. A sudden memory of their childhood flashed through the Doctor’s mind: the two of them, chasing each other around the TARDIS while their mother and her lovers watched and laughed at their childish antics. The Mistress had always been the more adventurous of the two, always running off and getting into trouble. As they’d got older, she’d shown signs of restlessness, growing tired of travelling to random planets and observing the people they found there. She’d never admitted it, not even to him, but the Doctor could tell that she wanted more.

One day she’d ran off and never returned. It wasn’t until their mother was dead and Rose and Jack were long gone that the Doctor ran into her again, and by then she’d changed so much that he hadn’t recognised her in her new regeneration.

But that was a long time ago, and he was still no closer to saving her.

“Leave,” she told him. “I don’t want to kill you.”

“Let me come with you,” he asked, almost pleadingly. “Wherever you’re going, we can go there together.”

She rolled her eyes and he was reminded again of their childhood. “As if I’d let you ruin my fun. We stopped playing together when we were 50.”

Frowning slightly, the Doctor figured it was probably OK to take a step closer. “Is that all this is to you? A game?”

“Don’t be so pedantic,” she snapped. “It was a figure of speech.”

He sighed. “Ro-Ro-”

“That’s not my name!” she snarled, turning back to the console and stabbing buttons furiously with black-painted fingers. With a lurch, the TARDIS began to move backward in time, moaning unhappily in their minds. She’d always hated it when they didn’t get along.

“Where are we going?” the Doctor asked, falling forward onto the console and clinging to it for dear life.

“You’ll see when we get there,” his sister told him, irritation plain in her voice. “Just before I kill you.”