Plot

On the most beautiful train in history, speeding amongst the stars of the future, a deadly and immortal creature is stalking the passengers.

The Doctor and Clara are on the most beautiful train in history speeding among the stars of the future. But a deadly creature is stalking the passengers. Once you see the horrifying Mummy you only have 66 seconds to live. No exceptions, no reprieve. As the Doctor races against the clock Clara sees him at his deadliest and most ruthless. Will he work out how to defeat the Mummy? Start the clock!—BBC America

Since the events of "Kill the Moon", and Clara's admission that she doesn't want to see the Doctor again, several weeks have passed. She allows him to take her on one "last hurrah". The Doctor takes her by TARDIS to a space-bound recreation of the Orient Express with passengers dressed in period costumes. The "train" is controlled by the computerised operator, Gus. While taking in the view, Clara confesses that she doesn't actually hate the Doctor, but cannot continue with him as before. They learn that an elderly woman, Mrs. Pitt, had recently died while claiming that she was attacked by a mummy that no one else could see. They retire to separate cabins for the evening. Clara calls Danny in her present and gets advice on how to properly end her relationship with the Doctor. She later encounters Maisie, Mrs. Pitt's granddaughter, who is distraught over the death and frustrated with the inability to see her body. They get trapped in the luggage car with a mummy's sarcophagus, and they bond while waiting for help. Meanwhile, the Doctor, claiming he is a mystery shopper, investigates Mrs. Pitt's death with the help of the train's engineer, Perkins. Perkins is also curious about the death, and the nature of the train. The Doctor speaks to Professor Moorhouse regarding the myth of the Foretold, a supernatural being who claims its victim 66 seconds after the lights flicker. The presence of the Foretold is further indicated when the train's chef dies in a manner similar to Mrs. Pitt's. The Doctor discovers Clara's situation, but when he tries to extricate her, the lights flicker and the sarcophagus opens. Before he can act, Captain Quell and his men arrest him for falsifying his credentials, despite his warning that someone is about to die. When the 66 seconds are up, they find that one of the Captain's men has died. The Captain, realising the Doctor was right, releases him. The Doctor begins to question what is really happening on the train. He realises that most of the passengers are scientific experts and demands to know why. The train suddenly stops in space. The illusory period decor of the original Orient Express and several of the passengers are revealed to be holograms when they vanish, revealing the remaining passengers to be in a laboratory. Gus tells them they will study the force behind the killings so that it can be reverse engineered as a weapon. Professor Moorhouse is the next victim, and he fearfully gives a visual description of the Foretold before he dies. The Doctor contacts Clara, who has discovered that the sarcophagus is meant as a containment unit for the Foretold. She also discovers records revealing previous attempts to discover the nature of the entity. Many previous ships and crews have been lost in the search for the entityin some cases, the force behind Gus purposely killed them when they failed to produce useful data on the entity. Gus warns the Doctor to end the call and return to work, and when he does not comply Gus decompresses the kitchen car, killing the kitchen staff. Gus threatens to kill more passengers and crew if the researchers continue their disobedience. The Doctor and Perkins discover that the past victims were all suffering from various medical conditions, and the Foretold is targeting the weakest. Captain Quell reveals he suffers from wartime post-traumatic stress disorder and soon sees the Foretold. He provides more information to the others before he dies: the mummy-like appearance, its ability to teleport, and how it appears out of phase with its surroundings. The Doctor and Perkins hypothesise that the Foretold drains the victim's energy through phase shifting, a process that takes just over a minute to complete. Perkins identifies the next likely victim: Maisie, due to her trauma over losing her grandmother. The Doctor tells Clara to bring Maisie to the lab after obtaining Gus' permission to unlock the storage door. On the way there, Clara discovers that the TARDIS is surrounded by a force field. She mentions this to the Doctor, and that Gus must know about him, the TARDIS, and his Time Lord nature to enable it to thwart their means of escape. The Doctor is forced to admit that Gus had been trying to bring him here to help for some time. Clara accuses him of purposely taking her into another dangerous situation. At this point, Maisie sees the Foretold. The Doctor absorbs some of her memories to trick the mummy into believing he is the intended victim. Within the 66 seconds, the Doctor determines that the Foretold is a former soldier from a war thousands of centuries ago, having been modified with phase-shifting camouflage to be an assassin. The Doctor offers their surrender to the Foretold, halting its attack. The Foretold becomes visible to everyone, salutes the Doctor, then disintegrates into dust with only its phase-shifting device remaining. Gus congratulates the passengers on their success. It then evacuates all the air aboard the train, since their services are no longer necessary. As Clara and the others pass out from lack of air, the Doctor takes the device and attempts to rewire it into a short-range teleporter Clara regains consciousness on a beach. The Doctor tells her he saved the passengers by teleporting them all into the TARDIS and then dropping them off at a city on a nearby civilised planet. His attempt to hack Gus to find out who was responsible caused the computer to destroy the train. Clara is pleased that the Doctor saved others this time, believing his heartlessness was a pretence. The Doctor admits that he was unsure that he could save Maisie, because he could not save Moorhouse or Quell. He explains that he would have used the data from her death, and any others, to defeat Gus. He gently explains to Clara that sometimes doing the right thing means facingand makinghard choices. Back on the TARDIS, the Doctor offers Perkins a job maintaining the time machine, having come to respect the engineer and expecting that Clara will be leaving. Perkins politely declines. Clara takes a call from Danny, who expects that she'll finally be ending her trips. However, she has now reconsidered her earlier decision and wants to continue her travels with the Doctor.