Stephen Carter was Lord Farthingale's secretary.
In 1920, Stephen was at Lord Farthingale's country retreat, when Sebastian Marten and Paul Steiffers were killed. He witnessed Lord Farthingale had possession of the key to the billiard room, which Sebastian was killed in.
During his investigation of these deaths, Dr Merrivale Carr gathered the house guests into the library, where he believed that the murderer was Kitty Kickshaw, who had taken the identity of Glenda Neil, and had taken the key. Immediately after, the Ninth Doctor and Rose arrived, explaining the deaths were the result of "invisible aliens".
A haze appeared, accompanied by a buzzing sound. Bart Faversham had started being lifted up and was tossed into the fire. The Doctor and Rose figured out that it was the drive of the invisible aliens' spaceship that was inadvertently causing this. The Doctor signalled the ship with the standard galactic code of the 455th century to repair the ship. The haze and buzzing subsided. (PROSE: Doctor vs Doctor)