"Dark Water" is the eleventh episode of the eighth series of the British science fiction television programme Doctor Who. It was first broadcast on BBC One on 1 November 2014. The episode was written by showrunner and head writer Steven Moffat and was directed by Rachel Talalay. It is the first of a two-part story; the concluding episode "Death in Heaven", the finale of the eighth series, aired on 8 November.

In the episode, Danny Pink (Samuel Anderson) is killed, and finds himself in an afterlife called the Nethersphere. As the Doctor (Peter Capaldi) and his companion Clara Oswald (Jenna Coleman) try to find Danny, they find themselves in a facility that accommodates Cybermen, run by recurring character Missy (Michelle Gomez). In the episode's climax, Missy reveals her true identity as a female incarnation of the Doctor's arch-enemy The Master, last seen in the 2009–10 serial "The End of Time".

The episode received widespread critical acclaim, with the performances of Capaldi, Gomez, Coleman and Anderson being praised. Critics praised the horror strand of the episode, the maturity level, and the darker, more mature and morbid themes.