Lieutenant Jeremy Carstairs was an officer in the British Army during World War I. He was abducted by the War Lords and hypnotised into participating in their simulated version of the war. He and Lady Jennifer Buckingham broke their condition thanks to the Second Doctor, and they assisted him in fighting against the War Lords. He, like all the surviving victims of the War Lords, was returned home by the Time Lords. (DW: The War Games)
The Doctor met Lieutenant Carstairs and Lady Jennifer again in 1915, though they had no memory of meeting him before. He then helped them and Winston Churchill escape from the Players who had captured them. Carstairs joined Churchill's staff, and he and Jennifer Buckingham eventually got married.
Carstairs remained in the Army after the war, transferring to Military Intelligence, and was eventually promoted to Colonel. In 1936, he aided Churchill, the Sixth Doctor and Peri in thwarting the Players' attempt to arrange for King Edward VIII to dismiss the government and establish a new one favouring Adolf Hitler, Carstairs coordinating his forces to keep an eye on people identified as Nazi sympathisers thanks to a list Peri had stolen from Joachim von Ribbentrop. (PDA: Players)
- Carstairs' first name was not given on-screen, only in the novelisation.