The Girl and the Monster

"The Girl and the Monster" is the first episode of the tenth season of Doctor Who and features the first appearance of the Slitheen in a Twelfth Doctor story. It also marks the first appearance of Christina.

Synopsis
"He's not a PM, he's a big fat green monster that can fit into human skin!" - The Doctor, about Fell Blotch Dreg

Seeking some recuperation, the Doctor arrives in modern day London and immediately gets involved in a Slitheen insurgent on Earth, and his love for a human woman.