As he was in an authority position within the business, Sim was noted as an "important individual" and was killed and replaced by a member of the Shoal of the Winter Harmony.
Using Sim's body, the alien continued to murder and replace other important figures, within the New York branch, including Mr Brock. When the Twelfth Doctor, Nardole and Lucy Fletcher discovered the truth he threatened to shoot them dead in "self-defence". However, the Ghost appeared and knocked Sim unconscious and saved the trio.
Due to Grant's supernatural abilities, Sim and Mr Brock prepared to murder and replace his brain. However, while they were after Grant, the Doctor and Nardole discovered their spaceship in orbit. The Doctor discovered that the spaceship was a rigged bomb and was able to work out that Sim and the other members of the Shoal were planning to destroy New York City. As the spaceship began falling towards Earth, Grant was able to stop it from impacting, thus preventing the destruction of New York.
UNIT then broke into the Harmony Shoal buildings all across the world, arresting those who were connected to the incident. However, in the New York branch, Sim's body was discovered empty of its alien host, which had travelled into a new host - a UNIT soldier. (TV: The Return of Doctor Mysterio)
Behind the scenes
Sim is one of Shoal of the Winter Harmony enemies in the Doctor Who: Legacy mobile game.