Morphant Master's possession of Bruce Gerhardt

Prior to his trial and execution on Skaro by the Daleks, the Master had planned ahead for this event, managing to survive via a Deathworm Morphant, and would subsequently possess the human body of Bruce Gerhardt.

Possession
The Doctor stored the Master's ashes in a casket and set his TARDIS on course for Gallifrey. However, en route, the Master's consciousness escaped from the casket and interfered with the TARDIS, causing a timing malfunction that resulted in an emergency landing in San Francisco during the final days of 1999. While the Doctor lay wounded after being caught in the crossfire of a gang war and was picked up by an ambulance, the Master exited the TARDIS via its keyhole and, (TV: Doctor Who) deciding that the Doctor was too injured, and the nearby boy, Chang Lee, too young, (PROSE: The Novel of the Film) he hid inside a bag belonging to the ambulance driver, Bruce. After Bruce had gone to home and bed, the Master forced his way into Bruce's body through his mouth, killing him and taking over his body. Bruce's eyes flickered open as the Master took full possession of his body. (TV: Doctor Who)

Aftermath
The Master only intended this body to be a stop gap, and his human body would begin to decay over time, with possession of the Doctor being his main aim. After killing Bruce's wife, he posed as him to learn what had become of the Doctor. (TV: Doctor Who) After passing through the Eye of Harmony, his body was re-energised, allowing him to continue to exist as that incarnation. (AUDIO: Day of the Master)