Doctor: Peter Capaldi (Twelfth Doctor)
Companion: Matt Lucas (Nardole)
Justin Chatwin – Grant Gordon / The Ghost
Charity Wakefield – Lucy Fletcher
Adetomiwa Edun – Mr Brock
Aleksandar Jovanovic – Dr Sim
Logan Hoffman – Young Grant
Daniel Lorente – Teen Grant
Sandra Teles – Reporter
Tanroh Ishida – Operator
Vaughn Johseph – Soldier
Directed by Ed Bazalgette
Written by Steven Moffat
Script editor Nick Lambon
Produced by Peter Bennett
Executive producer(s) Steven Moffat
Incidental music composer Murray Gold
Length 60 minutes
Originally broadcast 25 December 2016
In New York City on Christmas Eve of 1992, an 8-year-old boy named Grant wakes to find the Doctor dangling outside the window of his family's apartment and helps him come into his bedroom. Taking the boy to the rooftop, the Doctor reveals he accidentally set off a trap for a device he was building, and he enlists Grant to help complete it. However Grant mistakenly swallows a wish-granting gemstone needed for the device, believing it to be medicine, effectively being granted his wish to be a superhero. Abandoning the device, the Doctor makes Grant promise to not use his new superpowers before he leaves.
Returning to New York in 2016 with Nardole, whom he removed from Hydroflax's body and rebuilt, the Doctor investigates Harmony Shoals, a multinational research company. They encounter a news reporter, Lucy Fletcher, who is conducting a similar investigation. The group discovers that it is being secretly run by a group of living alien brains that transplant themselves into any living creature they need for their plans. They witness the killing of the company's owner, Mr Brock, for his body. Tracked down by Dr Sim, an employee the brains already took over, the group is suddenly rescued by a masked superhero named the Ghost, who afterwards transports Lucy close to her home. Returning to her apartment before her, the Ghost transforms back into Grant, who works for Lucy as a nanny. He is shocked to find the Doctor and Nardole waiting for him, having tracked him down via the gemstone fused inside his body.
When Lucy returns and sees the Time Lord, the Doctor reveals to her that the alien brains colonise planets by taking control of their prominent leaders, with Earth being their next target. Leaving Lucy to prepare for an interview with Grant's alter ego, the Doctor and Nardole track down the alien's ship in a low orbit and board it via the TARDIS. There, they discover that the ship's reactor is in a critical state and realise from Dr Sim that he intends to drop it on New York. Remembering an observation made by Nardole, the Doctor realises the city would be vaporised except for the Harmony Shoals building. World leaders would then take shelter in the company's other buildings within each capital city, believing Earth was under attack, effectively allowing the brains to take them over. After Dr Sim lets slip the ship is to be dropped at a designated time, the Doctor forces its descent ahead of schedule.Ghost and Lucy are captured by the brains during the interview. They plan to transplant themselves into Ghost's body. Ghost flees to deprive the brains of their prize, then returns as Grant to protect Lucy. Unable to change the ship's course during its descent, the Doctor sends Grant a message requesting his help. Grant manages to stop the alien's ship from crashing into the
Ghost and Lucy are captured by the brains during the interview. They plan to transplant themselves into Ghost's body. Ghost flees to deprive the brains of their prize, then returns as Grant to protect Lucy. Unable to change the ship's course during its descent, the Doctor sends Grant a message requesting his help. Grant manages to stop the alien's ship from crashing into the city, but reveals himself as the Ghost to Lucy in the process. Lucy is won over and Grant takes her in tow as he disposes of the ship. The Doctor alerts UNIT, which shuts down Harmony Shoals, unaware that the alien brain in Dr Sims has escaped within one of their soldiers.
Back at Lucy's apartment, Grant informs the Doctor he will no longer use his powers and retire his costumed identity. As the Time Lord leaves, Lucy asks him why he is sad. The Doctor gives only a vague answer before departing. Nardole privately reveals that the Doctor is mourning the loss of River Song, but will ultimately recover. He leaves with the Doctor to assist him further.