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Maisie Pitt was a passenger aboard the Orient Express. She was the granddaughter of Mrs Pitt, who preferred that Maisie call her “Mum”.
Maisie did not like her grandmother. She suspected that Mrs Pitt had poisoned her pony and the Twelfth Doctor later confirmed this.
While dining aboard the Express, Mrs Pitt complained to Maisie about a man dressed as a mummy. As she, and others in the carriage, were unable to see it, Maisie took it for illness and encouraged her to take medication. However, the elderly lady was killed by the being, a mythological mummy called the Foretold, with the cause being diagnosed as a heart attack.
Having often, half-heartedly, imagined the death of her grandmother, Maisie felt responsible for her passing. In a distressed state, she accused the Twelfth Doctor of lying when he claimed to have visited Thedion Four, as it had been destroyed thousands of years earlier. Captain Quell led her away, telling The Doctor and Clara Oswald that she was distressed following the death of Mrs Pitt.
During the night, Maisie attempted to gain access to the baggage train, where the body of her grandmother was being kept. She used a high-heel shoe to bypass security, breaking the electronic system that opened it to do so. With Clara, Maisie went into the baggage train, only to find the body absent. They then became locked in by Gus, the train’s controlling computer.
As they were imprisoned together in the baggage car, Maisie and Clara spent the night talking. They discussed not only Maisie’s relationship with her difficult grandmother, but also Clara’s recently challenging relationship with The Doctor. Maisie agreed with Clara that The Doctor had been wrong in his recent actions toward her, but suggested that Clara still cared about him.
Following the death of Quell at the hands of the Foretold, Maisie became its next target, selected due to her mental breakdown in the wake of her bereavement. The Doctor had Clara convince her to come to the laboratory. In order to prevent Gus from knowing his true intentions and potentially interfering, he lied to Clara, telling her he had no intention of saving Maisie, and simply needed to observe the details of how the Foretold killed her. He told Clara to lie to Maisie if she needed to do so. Clara reluctantly cooperated, and told Maisie that The Doctor thought he could save her.
Released by the computer, she made her way with Clara to the lab, where the Foretold appeared to her. The Doctor used a medical instrument to take her grief, torment and resentment, and imbued his own mind with it. The Foretold disappeared from Maisie’s view and appeared to him. Now able to access parts of her memory, The Doctor told her that her grandmother had indeed poisoned her pony and expressed sympathy for her father’s nature.
The Doctor identified the Foretold as a soldier and surrendered to it, releasing it to die. Gus then began removing the oxygen from the laboratory, causing the passengers to pass out while The Doctor worked to modify the teleport which the Foretold had been using. Clara woke up to find herself and the Doctor on a beach. He explained that he had saved the remaining passengers, including Maisie, by teleporting them to the TARDIS, and then dropping them off at the nearest populated world. (Mummy on the Orient Express)