The Doctor (Peter Capaldi) discovers that something is draining energy from the TARDIS, and it materializes in Bristol. Clara meets Rigsy (Julian Wade), a graffiti artist assigned to community service on a council estate. He tells her about several people who have gone missing. When Clara returns to the TARDIS, she finds its outside has shrunk, though the inside is still enormous, and the Doctor can’t leave. He passes out his sonic screwdriver and psychic paper, and a communication earpiece. She sticks the TARDIS in her tote bag.
Clara convinces P.C. Forrest (Jessica Hayles) to let her and Rigsy into the flat of the first person to disappear. They hear Forrest scream and see no sign of her except a mural on the wall. The Doctor recognizes it as a human nervous system. He thinks this is the work of two-dimensional creatures called the Boneless, flattening people into two dimensions so they can invade. Clara and Rigsy escape.
They go to the other community service crewmen to warn them that the murals in a pedestrian subway are alive. Some are killed while Clara leads the others through a train yard into tunnels. The Boneless follow them in the flickering form of the people they have killed. They surround the others by flattening the doors into two dimensions. The Doctor creates a device to reverse this. Clara drops the TARDIS onto the train tracks. The Doctor switches on Siege Mode so there is no damage when it is struck by a train, but there is no power left to reverse it.
Clara has Rigsy paint a door on a large poster, which they hang over an access tunnel. The Boneless think it is a door and funnel their energy into it, but it powers the TARDIS instead, which reverts to its normal size. He returns the Boneless to their dimension. Missy watches them.
The episode was admired as clever and outlandishly original, and it was praised for the pairing of the Doctor and Clara and their complex relationship. The character of Rigsy was based on Banksy. This is the first time since the classic series that the Doctor opens the doors using the control panel. When it is in Siege Mode, the TARDIS becomes a miniature Pandorica. This was the 250th Doctor Who story. It was influenced by the 1884 novella Flatland. At one point, the Doctor sticks his hand out through the door and moves the TARDIS like the disembodied hand called Thing on the Addams Family. The train is the A113, named after the classroom at the California Institute of the Arts where the Pixar artists came from. I really loved this one. The Doctor peeking out of the tiny TARDIS doors, then passing objects out through the opening was startling and made me laugh every time.