Parts: 1 (60-minute)
Writer: Chris Chibnall
Airdate: 07 October 2018
Production Code: n/a

Doctors: thirteenth
Companions: Ryan, Yaz, Graham

Story: While trying to recover his bike in a Sheffield forest, 19-year-old Ryan is startled by the sudden appearance of a large, bulbous iridiscent object that's freezing to the touch. Local police, in the form of his childhood friend Yasmin Khan, are unable to provide much help... but things get even weirder when Ryan receives a frantic call from his nan, who's stuck with her husband on the train, stopped in the middle of the countryside with all the exterior doors locked, as something mysterious and unnerving slithers through the dark train cars toward them.

Before Ryan and Yaz can reach his nan, help arrives in the most unexpected fashion: a blond woman plummets through the roof of the train, and helps to fend off the bizarre entity. But what did it want with them in the first place? Why did it suddenly leave? And how is it connected to the bulbous object in the forest? The key to finding these answers is the woman who fell out of the sky, if only she can remember who she is.