Terrence Malson was a billionaire who transferred his mind into a small child named Jeff Smith to prolong his life.
When the boy would turn sixteen, Malson would be able to control Jeff's body. However, at the suggestion of the Eighth Doctor, Jeff kept erasing his own memory to prevent Malson from taking control. Jeff eventually shot himself in the head to prevent Malson from taking him over. Malson seemingly disappeared soon afterwards. (AUDIO: Running Out of Time)