The Doctor's Big Plan - Doctor Who Season 9 Episode 3

The Doctor decides to go back before the flood to stop the events that created the killer "ghosts."
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Underwater and Under Attack - Doctor Who

The Doctor and Clara devise a plan to save the base's frightened crew from an unknown homicidal force.
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Doctor Who Season 9 Episode 3 Quotes

The Doctor: Tell me, what about those things out there? What are they? Why are they trying to kill us?
Bennett: Well, they're... they're ghosts.
The Doctor: They're not ghosts.

We are fighting an unknown homicidal force, underwater, in a nuclear reactor.

The Doctor