Clara emerged, looking a bit woozy. The Doctor brought her up to speed. Vastra explained that her people had a bitter enemy when they ruled the world: The Red Leech, a parasite that secreted a fatal poison. It had returned in Victorian Times as the Crimson Terror. Clara suggested they investigate the chimney, where they found Mrs. Gillyflower's rocket: She was planning to poison the air with the poisonous red substance. The old dame even had a cool piano-machine-slash-secret steampunk launch controls.
The Doctor found Ada crying, and took pity on her. He thanked her for saving her life, and insisted that her mother was not to be trusted. "I need you to tell me something," said the Doctor. "Who is Mr. Sweet?" She could not betray her mother, even after everything. The Doctor insisted that she join his squad: "There's something you need to know."
The Doctor and Clara faced off against Mrs. Gillyflower. "Mr. Sweet is always with us," she explained, noting that she had a very close relationship with her silent partner. Symbiotic, in fact…at which point she revealed a terrifying red lobster-leech-parasite hiding on her chest, with cold black eyes and a wide variety of sharp bits. (It wasn't quite as gross as the reveal of Kuato in Total Recall, but consider: This was Diana Freaking Rigg.)
"He's grown fat on the filth that humanity has pumped into the rivers," she explained -- a monster from prehistoric times, brought back to life by modern pollution, who gave her his venom in exchange for sustenance. "Mrs. Gillyflower, in the wrong hands, that venom could destroy the planet." She held out her hands, and delivered the line of the episode: "Do you know what these are? The wrong hands! Haha!"
She explained her plan: To explode the rocket in the atmosphere and destroy all of humanity. Her new Adam and Eves would step out into "a new golden dawn." The Doctor changed the subject: "Tell us about Ada." The story about Ada's drunk father was all bunk. Mrs. Gillyflower had experimented on her daughter, to discover an anti-toxin -- she had left her daughter blinded, covered in scars. Ada stepped in: "You perfidious hag! You piranha! You harpy!" She began to beat her mother with her cane. Meanwhile, Clara took advantage of the moment to cripple the rocket-launch controls with a high-tech weapon: A chair.
Mrs. Gillyflower led her daughter out at gunpoint. The Doctor and Clara raced to catch up with her at the rocket launchpad. (ASIDE: Am I crazy, or did the whole look of this last scene -- the tower, the stairs, the person falling to their death -- not-so-vaguely resemble a key scene from Vertigo? Could it be that the Doctor somehow brought Clara back to life in three time periods -- or rather, inadvertently caused the soul of Clara to inhabit the bodies of three different people? Am I just reaching? END OF ASIDE.) Mrs. Gillyflower made her way to the secondary firing mechanism, and fired the rocket. All was lost -- but the Doctor had friends. Vastra and Jenny had removed the red poison.
NEXT: And she would've gotten away with it, if it weren't for you meddling kids