Flatline was the ninth episode of the eighth series of Doctor Who produced by BBC Wales. For scheduling reasons, it was a “Doctor-lite” episode.
Flatline was Jamie Mathieson’s first script for the show. Two years before writing it, he had pitched his ideas for a story to Steven Moffat, but was unsuccessful. When he again met with the executive producer, he showed him four ideas for episodes, complete with his own illustrations, aided by his background in art college. Taking an interest in the monster he had created for what would become Flatline, Moffat asked Mathieson to produce a story outline and he got the job to script the episode.
It was only after writing several drafts that he was told the episode would need to lock the Doctor away in a single location, as Peter Capaldi’s scenes for the episode needed to be filmed quickly to bide by the production schedule.
Mathieson decided to write a script where the Doctor was “in the dark”. For this to be successful, he had to create a unknown quantity to feature as his alien enemy. Much like his next creation, the Foretold, he elected to have no dialogue for the aggressors, allowing something about them to remain “unknowable”.
The Twelfth Doctor misjudges the location when returning Clara home, resulting in them landing in Bristol rather than London. They also discover that the exterior of the TARDIS has shrunk down due to its power being drained by something nearby; the Doctor stays with the smaller TARDIS to figure out what is going on while Clara looks around. She finds that locals are disappearing, and mystery murals of them painted in a nearby pedestrian tunnel. Clara returns to find the TARDIS’ exterior has shrunk further, trapping the Doctor within as he is unable to fit through the miniaturised doors. He hands Clara his psychic paper, the sonic screwdriver, and an audio/visual earpiece so he can keep in touch with her, and then has her carry the TARDIS with her to where he has tracked a large source of energy.
Clara poses as “Doctor Oswald” and gains the confidence of a local graffiti artist, Rigsy, who is part of a community service team, despite the Doctor’s initial hesitation towards him. After finding nothing but a strange desert mural at the flat of the latest disappearance, Rigsy gains help from a police officer to gain access to the flat of the first known missing person. The Doctor instructs Clara to tear out the walls, believing the energy source to be within them. While they work, PC Forrest is sucked into the ground out of view of Clara and Rigsy, and when they reach her screams she has disappeared. They look around the room and find another strange mural, and the Doctor realizes it’s likely to be PC Forrest’s nervous system, and believes the first mural was a closeup of human skin of the latest victim. As the entities start to move towards Clara and Rigsy, the Doctor quickly explains his theory that their adversaries are creatures from a two-dimensional universe, and must have been experimenting on the three-dimensional world, trying to understand it. As Clara and Rigsy try to avoid the entity, which is in the walls and ground, Danny phones Clara, and she manages to avoid questions of her whereabouts and quickly hangs up. The Doctor, who is listening in, recognizes that Clara has lied to him about Danny accepting her continued travels in the TARDIS.
Clara and Rigsy return to the rest of the community service group, who are about to paint over the murals in the pedestrian tunnel. The
Doctor realizes that the images are masquerades for the two-dimensional creatures, attempting to understand three dimensions; after the creatures take one of the services workers, Clara leads the rest away with the creatures chasing them. They take shelter in a nearby engine repair warehouse, where the Doctor helps Clara to try to communicate with the creatures using mathematics. When another worker is taken, Clara and the surviving members flee into a disused subway tunnel. As they explore, they find their only escape route has been flattened to two dimensions by the creatures. The Doctor provides Clara with a device to restore the dimensions, and they just manage to escape. The Doctor also tells Clara that he has worked out a way to stop the creatures and return them to their home dimension, but the TARDIS doesn’t have enough power. In the chaos, the TARDIS is knocked down a shaft on to an active railway line, with an oncoming train heading right for the TARDIS and the Doctor. With some advice from Clara, the Doctor is able to get the TARDIS off the rails by using his hand to move it, resulting in the Doctor doing a little victory dance for saving his TARDIS and himself. However, his victory is short-lived, as the TARDIS tips over onto the rails again. In order to protect the TARDIS from the oncoming train, the Doctor activates its siege mode at the very last minute, but without power is unable to return it to normal. The lack of power is also causing the life support systems to fail. Unsure if Clara can still hear him or even if she is still alive, the Doctor praises her performance, saying that she “Made a mighty fine Doctor.”
Clara uses the sonic screwdriver to stop an out-of-service train, and they attempt to use it to ram the creatures to prolong time to get back in contact with the Doctor. However, the train is simply transformed into two dimensions; as they escape, Clara finds the TARDIS now looking like a plain cube with Gallifreyan symbols, and takes it with her. Taking shelter in a disused office space, Clara comes up with a plan to provide energy to the TARDIS by having Rigsy paint out a fake access door on the back of a large poster. When the creatures attempt to pull it into three dimensions, they instead feed their energy into the TARDIS, restoring it to normal. Realizing they have no interest in peace, the Doctor uses the sonic screwdriver to stop the creatures, whom he dubs the “Boneless”, sending them back to their own dimension with a warning to those who survive the trip that “You are not welcome here. This plane is protected. I am the Doctor.” The Doctor returns everyone to the surface safely in the TARDIS. Clara rejects a call from Danny, catching the Doctor’s attention; he notes that she enjoyed ‘playing the Doctor’ for the day. She asks him to admit that she was a good Doctor; he reluctantly tells her that she made an “exceptional Doctor,” but that “goodness” had nothing to do with it.
As Clara watches the Doctor enter the TARDIS, Missy, seated in a darkened room, is watching Clara on what appears to be a tablet. Missy says, with regard to “her” Clara, that she has “chosen well”.