Mrs Gillyflower tucked herself into one of the alcoves, shielding herself from the flames, as the missile thundered upwards. She beamed in triumph, her complexion glowing crimson in the light from her terrible creation. Even Mr Sweet took note, his blackcurrant eyes like little points of fire as he craned his serpentine neck to watch.

Within moments, the rocket rose from its encasing chimney and shot into the night sky, leaving a fiery trail like a comet.

Cackling with joy, Mrs Gillyflower stroked the sticky flesh of her symbiotic friend. ‘Now, Mr Sweet, now the whole world will taste your lethal kiss!’

And then, amidst the dying fall of the rocket’s engines as it sailed heavenwards, the small, quiet voice of the Doctor: ‘I don’t think so.’

He was pointing one bony finger towards the other side of the chimney. Mrs Gillyflower turned. As the smoke cleared, two black-bonneted Pilgrims were revealed, framed there like widows at a drear funeral. But pulling back their bonnets they revealed themselves to be Miss Jenny Flint and Madam Vastra! And on the ledge next to them sat the carboy of glowing crimson venom – never loaded!

Mrs Gillyflower’s withered countenance fell, like a deflating linen wash-bag. Then a new, even madder and more dangerous glint appeared in her eye. She raised her pistol and pointed it at the carboy.

‘Very well, then. If I can’t take the world with me – you will have to do. Die! All of you … freaks! Die! Die! DIE!’

Her finger tightened on the trigger.

All of a sudden, there was Strax, upside down on a rope, dangling from the lip of the chimney like some circus aerialist, his strange alien gun raised!

‘Put down your weapon, Human fe-male!’ he barked.

‘Never!’

She raised her pistol – and Strax fired. A bolt like white lightning struck the stair at Mrs Gillyflower’s feet and she stumbled – fatally. In an instant she was falling down, down, down the empty chimney, landing at its base with a fearful crack and spayed there, arms akimbo, like some broken china doll.

Tap, tap, tap.

Cane in hand, Ada made her way from the bottom of the chimney stair towards the crumpled heap of skirts which was her mother. Behind her, the Doctor, Clara, Jenny, Madam Vastra and Strax slowly descended, keeping a respectful distance.

Mrs Gillyflower’s eyelids fluttered. And then the repulsive red leech began to disentangle itself from her neck, its horrible legs wrenching themselves from her flesh.

‘No! No … Mr Sweet? Where are you going?’ wailed Mrs Gillyflower. ‘You can’t leave me now, Mr Sweet? MR SWEET!’

Clara looked on in disgust. ‘What’s it doing?’

‘It knows she’s dying,’ said the Doctor, pityingly. ‘She’s no longer any use to it.’

The vile creature began to slither and click its way over the Mill floor, leaving a trail of sticky red slime.

Mrs Gillyflower held out a withered claw. ‘Ada? Ada … are you there?’