On Christmas Eve, 1992, in New York City, an 8-year-old boy named Gordon Grant (Logan Hoffmann) wakes up to find the Doctor (Peter Capaldi) hanging on the building outside his window and helps him into his bedroom. They head up to the roof and the Doctor, who Grant calls Doctor Mysterio, explains that he accidentally sprung a trap on a device he was building. Grant helps him finish it, but there is a wish-granting gemstone inside and Grant, thinking it’s medicine, accidentally swallows it, while wishing to be a superhero someday. The Doctor gives up on the device and leaves.

In 2016, the Doctor returns to New York with his cyborg companion Nardole (Matt Lucas) to investigate a multinational company called Harmony Shoals. A news reporter named Lucy Fletcher (Charity Wakefield) is doing the same. The company is run by living alien brains that transplant themselves into any living creature. They see the company’s owner, Mr. Brock (Adetomiwa Edun), killed for his body. Dr. Sim (Aleksandar Jovanovic) pursues them, but they are rescued by a masked superhero called The Ghost (Justin Chatwin), who is Grant Gordon grown up. He is working for reporter Lucy Fletcher as a nanny. The gemstone is still in his body, and he is tracked down by the Doctor and Nardole.

Lucy returns and interrogates the Doctor, who tells her the aliens are trying to control the Earth by taking over its leaders. The Doctor and Nardole leave Lucy to interview the Ghost and infiltrate the aliens’ ship with the TARDIS. The reactor is going critical and the aliens plan to drop it on New York. The only survivors of the attack would be the Harmony Shoals buildings in each capital city and the world leaders would shelter there, ripe for the picking. The Doctor fixes the reactor so it will be released early.

The Ghost and Lucy are captured by the aliens. The Ghost flees and returns as Grant. He stops the ship from crashing, revealing himself as the Ghost to Lucy. The Doctor calls UNIT, which shuts down Harmony Shoals. Grant tells the Doctor that the Ghost will be retired.

The device the Doctor is building is supposed to reverse the time-paradox that prevents him from visiting Amy Pond and Rory. The aliens appeared in the previous episode, where they were the servants of King Hydroflax. Three comedians have played companions: Catherine Tate as Donna Noble, James Corden as Craig Owens, and Matt Lucas as Nardole. The Husbands of River Song was also a Christmas special. This was the first time that two Christmas episodes were broadcast on Christmas Day. Doctor Mysterio is the name of the Doctor Who serials in Mexico. The story was praised as a “romp with a classic feel.”

The story is full of nods to Superman and other superheroes. The names Siegel and Shuster are mentioned. The Harmony Shoals Building is topped with a globe like the Daily Planet Building. Grant is referred to as mild mannered, and he has roughly the same powers as Superman. He tears open his shirt when he leaps into action. Lucy interviews him on a rooftop. Lucy’s married name is Lucy Lombard, giving her the initials of LL. There are Spiderman and other Marvel posters on the walls of young Grant’s bedroom. The Ghost refers to The Phantom.