In the luxury dining car of a train, Mrs. Pitt (Janet Henfrey) sees a man dressed as a mummy and a 66-second clock starts at the bottom of the screen. She asks her granddaughter Maisie (Daisy Beaumont) if there’s a costume party going on, but Maisie can’t see him. The guard can’t see him either. As the clock ticks down, the mummy approaches closer and closer to Mrs. Pitt, and when it reaches her, she dies.

Clara (Jenna Coleman) allows the Doctor (Peter Capaldi) to take her aboard a space-bound train modelled after the Orient Express. She does not know that Gus (voice of John Sessions) the train’s computer, has invited the Doctor along with many scientists. The Doctor hears about the Mummy and becomes curious. It seems that the death of Mrs. Pitt, and other similar deaths, all took place after 66 seconds following a flickering of the lights. This is similar to a legend called the Foretold.

When the Doctor becomes suspicious of all these scientists, the train disappears, revealing a laboratory. Gus informs the Doctor that the passengers must study the Foretold to understand its technology. The sarcophagus in the storage car, discovered by Clara and Maisie, is for capturing the Foretold. Professor Moorhouse (Christopher Villiers) and Captain Quell (David Bammer) are next to die.

The Doctor and Chief Engineer Perkins (Frank Skinner) discover that the Foretold is draining energy from its victims with phase-shifting technology. Also, the victims were always the medically weakest on the train. Perkins discovers that Maisie is probably next. Clara brings her to the lab. Maisie sees the Foretold and the Doctor tricks it into targeting him, using Maisie’s memories. As 66 seconds pass, the Doctor identifies the Foretold as a modified stealth soldier of an ancient war. He surrenders to it, ending the war. The Foretold collapses into a pile of dust. The Doctor takes everyone away in the TARDIS. The computer Gus self-destructed to keep its truth hidden. Clara lies to the Doctor, saying that Danny is okay with her travelling in the TARDIS.

The episode was well-received, the well-made mummy thought to be scary and the train corridors claustrophobic. It was voted best of the season by Doctor Who Magazine. It was Peter Capaldi’s idea to have a cigarillo case with jelly babies in it. At one point, he says, “Are you my mummy?” A singer does a jazz version of “Don’t Stop Now” by Queen. Writer Jamie Matheson based Perkins on a train-buff friend who told him all about the Orient Express.