With the Doctor trapped, Clara must fight off a menace from another dimension while trying to free him. 

A man calls the police. They are everywhere, he says, all around. We've been so blind. He is suddenly ripped away and disappears. Or does he? His face is ingrained in the woodwork around the wall as he's screaming.

When The Doctor tries to return Clara to her same time same place (ish) the TARDIS has shrunken. And they're in Bristol. Hmmm.

Flash to people doing community service. In particular, a young man who was a tagger is being asked to paint over his signature.

There is a memorial just outside a tunnel and inside the tunnel is a mural of people who have gone missing. Interestingly, their portraits are full size as if they're stepping into the wall.

Clara goes back to the TARDIS to discover it is now toy sized. Clara laughs at him when his big ole face is there. 

Clara is The Doctor for a while. She loves it and she's pretty good at it. Clara believes that it may be related to the shrink ray. Or, as The Doctor says... in the walls.

The walls have groovy wallpaper. The cop knows something is happening, but she doesn't know what it is but she is sucked into the floor by it.

Clara uses the sonic screwdriver to toss her and Rigsy out the window and out of the house. The aliens are from a two dimensional universe, long been believed but not proven. As Stan gets stuck into the wall, the portraits turn around. They're wearing the dead like camouflage. 

Clara assumes what The Doctor would do. Give them hope. People with hope run faster than those who think they're doomed. Thus, you lie to try for the best possible outcome.

Even the TARDIS is doomed because it's unsure if the flattening things is being done as a way to communicate. Perhaps they don't even know we need three dimensions to communicate. 

The Doctor creates something that will return two dimensions to three, and he called it the 2dis. Get it?

The aliens have learned to be 3D. They're out and running about!

The old man dumps The Doctor down a hole. he'll be of no help. Still, he thrusts his hand out of the TARDIS to crawl across the train tracks. He celebrates, until he falls over on the tracks.

Clara uses the screwdriver to top the train. 

Rigsy decides to man the train to save them, but Clara uses her hairband instead.

The creatures made it through the train, but Clara finds the TARDIS which is now a perfect cube.

Clara commissions a graffiti from Rigsby. 

Life support inside the TARIDS is failing. Because the creatures can't tell the difference between two and three dimensions, a graffiti door forces them to try to redimensionalize it, but instead they recharge the TARDIS through the wall.

The Doctor realizes that the monsters know what they're doing and he's doing his job. This plane is protected. He is The Doctor and he thrusts them back to their own dimension.

Clara takes control over the situation and when the TARDIS is restored, the world is saved. Yet The Doctor isn't happy. Why?