Plummer was a vicar and a personal confessor for a famous celebrity. The Fifth Doctor tasked him with piloting a spaceship once its pilots were killed. (AUDIO: Lightspeed)
The Doctor would eventually discover that Plummer, along with many other people throughout history who looked like him, were caused by a group of Time War Daleks who — while on a mission to prevent the rise of Time Lord society on Gallifrey — created a time distortion which created alternate versions of themselves as four un-mutated Kaled people. This time distortion also caused there to be multiple copies of these four Kaleds at various points throughout time and space. (AUDIO: Effect and Cause)