The Doctor took his turnip-sized fob watch from his waistcoat. ‘I think she’s about done.’

So saying, he creaked open the lid of the machine and, through the pall of steam, Clara’s pretty face was visible. She blinked, taking us all in.

Madam opened her mouth to speak, but the Doctor got there first. ‘I know who you think she is. But she isn’t. She can’t be.’

‘I was right, then,’ breathed Madam. ‘You and Clara have unfinished business.’

The Doctor bent his face close to Clara and smiled reassuringly. ‘Hello, stranger.’

‘Doctor?’ She coughed, unused to speaking. Unused to doing anything much for a while.

‘What’s … what’s going on?’

‘Haven’t you heard?’ said the Doctor with rather more excitement than was quite decent. ‘There’s trouble at t’mill!’ And then he was off again, like a jack-in-a-box-in-a-frockcoat, tearing down the corridor towards the lift.

I helped the pasty-faced Clara (if it was Clara) along as Madam Vastra struggled to keep up with the Doctor, her skirts swishing on the wooden floor.

‘I knew that I recognised the symptoms,’ she said. ‘The glowing red flesh. It’s unmistakable.’

‘It is?’ the Doctor said.

Clara was looking at me and a frown creased her pretty face. She swivelled her eyes towards Madam and her frown deepened. ‘H-hi,’ she managed.

‘Hello,’ I said.

‘Hello,’ said Madam. Then she returned to the Doctor who was busy summoning the lift from below. ‘My people once ruled this world, as well you know. But we did not rule it alone. Just as Humanity fights a daily battle against nature, so did we. And the greatest plague, our most virulent enemy was the repulsive red leech!’

‘Ooh! The Repulsive Red Leech!’ cooed the Doctor. Then his face fell a bit. ‘Nah. On balance, I think I prefer the Crimson Horror. What was it exactly?’

‘A tiny parasite. It infected our drinking water. And once in our systems, it secreted a fatal poison. Perhaps it survived all these millions of years, just as my people did.’

‘If it’s been hanging around, lurking in the shadows, maybe it’s evolved,’ mused the Doctor. ‘Or maybe it’s had help.’

‘Doctor.’ It was Clara, piping up at last, her voice still croaky and unused. ‘Doctor, I’ve been thinking. The chimney—’

‘Yeah, yeah, yeah. Way past that now!’ he cried. ‘Yucky red parasite from the time of the dinosaurs pitches up in Victorian Yorkshire. Didn’t see that one coming!’

‘But the chimney—’ Clara insisted.

‘But what’s the connection to Mrs Gillyflower? “Judgement will rain down on us all … ”’ The Doctor looked round and it was like you could hear the cogs whirring in his brain. ‘An empty mill … ’ he whispered.

‘And a chimney,’ said Clara, grabbing him gently by his substantial chin and twisting his face round towards her, ‘that doesn’t blow smoke … ’

Both their faces turned to look up then the Doctor beamed. ‘Clever clogs.’