Doctor: Peter Capaldi (Twelfth Doctor)
Pearl Mackie (Bill Potts)
Matt Lucas (Nardole)
Jennifer Hennessy – Moira
Stephanie Hyam – Heather
Nicholas Briggs – Voice of the Daleks
Directed by Lawrence Gough
Written by Steven Moffat
Script editor Nick Lambon
Produced by Peter Bennett
Executive producer(s) Steven Moffat
Incidental music composer Murray Gold
Series Series 10
Length 50 minutes
Originally broadcast 15 April 2017
The Doctor and Nardole are living under the guises of a university professor and his assistant. Bill Potts, who works at the university's canteen, is called to the Doctor's office, where he notes she attends all his lectures, and Bill eventually becomes the Doctor's student.
Months pass, and Bill develops a mutual attraction to a student named Heather, who has a defect that makes it look like there is a star in her eye. Heather asks her to inspect a mysterious puddle, asking her if she can see what's wrong with her reflection, but Bill does not notice anything except that her face seems wrong somehow. Bill turns around and Heather has left. Within the puddle, a voice proclaims that a search for a pilot has begun.
Bill meets Heather again, who encourages her to look into the puddle once more, then apparently vanishes. The Doctor investigates, and notes that the reflection is not a reflection, but something mimicking them. Bill returns to her flat, and is chased by a moving body of fluid, which takes Heather's form. She runs to the Doctor's office, where fluid-Heather materialises, forcing him and Bill into the TARDIS. The Doctor moves the TARDIS elsewhere in the university, where he and Nardole check on a vault they have been guarding. They conclude that fluid-Heather is only interested in Bill just as the creature appears, causing the trio to flee in the TARDIS.
The TARDIS lands again on a boat in Sydney Harbour facing the Sydney Opera House, where the Doctor admits that he is an alien. The fluid-Heather arrives again, having travelled the planet in just a minute. The Doctor takes them to another planet, millions of years into the future, but she finds them again. The Doctor speculates that the substance is a living fluid from an alien ship, and that it made Heather its "pilot" because she wanted to run away. He decides to have it chase them through a battle between the Daleks and the Movellans. The water-Heather assumes a Dalek's likeness after taking a shot meant for the Doctor and Bill. Bill convinces "Heather" - who wants to make her a passenger - to let her go, and it departs.
The Doctor and Bill return to his university office, where he attempts to wipe Bill's memories of the day's events, but she stops him. He says that he has promises to keep, that he is in disguise and no-one should know who he is. He changes his mind, however, and as Bill leaves the grounds she finds him waiting for her with the TARDIS. She agrees to travel with the Doctor.