"Flatline" is the ninth episode of the eighth series of the British science fiction television programme Doctor Who, written by Jamie Mathieson, and directed by Douglas Mackinnon. The episode stars Peter Capaldi and Jenna Coleman, with Joivan Wade and Christopher Fairbank guest starring. The episode received critical acclaim, with particular praise directed at Coleman's performance.

On returning Clara Oswald to London, the Doctor finds that something is draining energy from the TARDIS and they end up in Bristol instead. They find the TARDIS' exterior size has shrunk as a result of the energy drain, and the Doctor sends Clara out to learn more while he tries to stop the drainage. Clara befriends Rigsy, a graffiti artist now performing community service with a small crew. Rigsy informs her that several people have gone missing recently, their disappearances depicted on a mural in a pedestrian tunnel painted by an unknown person. Clara returns to find that the TARDIS has now shrunk so much that the Doctor cannot come out of its door. He passes her his sonic screwdriver, psychic paper, and an earpiece to let her communicate with him, and asks her to carry the TARDIS with her as she rejoins Rigsy, taking on the role of "Doctor Oswald" and adopting several of the Doctor's mannerisms.

At the flat of the latest disappearance they find nothing amiss save for a strange mural of a desert on one wall. With the help of police constable Forrest, they enter the flat of the first disappearance, and the Doctor instructs them to tear out a wall as he suspects that the source is within. In the next room, PC Forrest is suddenly absorbed into the floor, and by the time Clara and Rigsy arrive, she is gone. The Doctor finally realises that the murals are in the shape of a human nervous system or a human skin and that the attacks come from creatures, which he nicknames Boneless, that live in two dimensions and are experimenting to understand the third dimension. Clara and Rigsy manage to escape through a window using a hanging chair. During the escape, Danny Pink calls Clara as she is late for their meeting in London. The Doctor overhears the call and knows that Clara has lied, both to Danny about the nature of her travels with the Doctor, and to the Doctor in affirming Danny's acceptance of her traveling with the Doctor.

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