The episode starts out harmlessly enough, with the Doctor taking Clara home. Clara hasn't told Danny that she's continuing to travel with The Doctor and she's also lying to The Doctor about Danny being fine with their adventures. It's hard to see why Clara's bothering with this juggling act since Danny's clearly too smart to fall for Clara's really terrible lies, it starts to fall apart when Danny calls her at an inopportune time. "Does it even count as lying if you do it for somebody's own good?" Clara asks. Yes, Clara, it does, especially since Danny has already confronted you about being honest with him. This really can't end well.
However, the question of Danny is derailed when the TARDIS veers off course and lands in Bristol. While these days The Doctor is shown as mostly in control of the TARDIS, this setup is reminiscent of countless episodes of classic Who in which the week's adventure only occurred because the TARDIS didn't land where intended -- and when the TARDIS is steering, it has a keen nose for trouble. It turns out people have been disappearing in town -- including some genuine locked-door mysteries, which piques The Doctor's interest -- but more than that, the TARDIS has shrunk. "This is huge!" The Doctor enthuses while walking around a chest-high TARDIS... and of course they can't go anywhere until they figure out what's wrong.
But the real twist comes a bit later, after the TARDIS shrinks again -- now toy-sized -- trapping The Doctor inside. This means Clara needs to be his eyes and ears for the episode, and he passes her the sonic screwdriver and the psychic paper through the TARDIS door to arm her for her investigation. (There's an Alice in Wonderland quality to the tiny TARDIS -- especially when The Doctor is talking to Clara or passing her things through the door -- which adds to the episode's great visuals.) Clara tucks the TARDIS in her purse and spends most of the episode wondering what The Doctor would do in her situation -- at times mimicking him (calling herself Doctor Oswald) and at other times openly mocking him... but, for the most part, it's fun to watch Clara investigating while the Doctor's stuck on his own, only able to offer color commentary to Clara through an earpiece.
But as the episode's stakes rise, the laughs become fewer and further between. The monster of the week -- which The Doctor dubs The Boneless -- is determined to be something from beyond the known universe that only exists in two dimensions. When it attacks, it turns its victims into 2D "art" of a sort, which explains graffiti around town of the missing people. When attacking, the creature appears almost like a liquid -- but really just a flat color moving across walls or floors -- and when it reaches its target, they, too, become two dimensional, turning into a seeming splatter of paint on the walls. As the creatures learn more about their environment, they manage to turn themselves into blocky 3D representations based off of the people they've attacked -- and whether they're in 2D or 3D, they're thoroughly frightening as they hunt down Clara and the group she's fallen in with. The Boneless turn people into 2D paintings without warning... and sometimes the group only notices when they see the now-2D person from the wrong angle. Creepy.