What. An. Episode. This episode not only stood up to the quality of the sensational The Magician's Apprentice from last week, but it managed to surpass it altogether, a rare act for a double-part story these days. The episode toned down the epic tone of its predecessor, to focus almost entirely on two relationships, Clara and Missy, and the Doctor and Davros. The acting from the entire cast - especially Bleach and Capaldi - was incredible, and the talks between the Doctor and Davros took the ancient enemies into completely new - and unexpected - territory, dripping with surprising emotion, powerful dialogue, and truly touching moments. Elsewhere, Missy is devilishly evil and proves herself to be one of the best incarnations of the Master yet. One second, she's being hilarious and bouncing off Clara brilliantly, the next you literally hate her very despicable guts. She's funny and electric, but damn you can't trust her. And, ultimately, the crux of this episode is in its optimistic portrayal of the concept of mercy and compassion, and simply how powerful was the final scene, the Doctor and the young Davros, hand in hand, Davros carrying the screwdriver and the Doctor carrying the Dalek gun? What a powerful image, concluding what is by far one of the best Doctor Who episodes ever told. Superlatives all round for this one; Moffat, this may be your greatest ever script for the show. Hopefully the rest of Series 9 can keep to this astounding quality.