Simon was model 1711 "Robohuman" who persuaded a criminal named Eryk Zang to help him in his "researches". Simon decapitated Eryk and connected Eryk's brain to his own cognitive processor, in search of something organic that he felt was missing in himself.
He called in the Sixth Doctor and Mel because he thought he was experiencing dreams. The Doctor thought it wasn't possible, but agreed to examine him. He persuaded Simon to go offline for ten minutes, and he and Mel started investigating Simon's apartment. However, Simon had only gone offline for eight minutes. He tried to justify to the Doctor what he had done. He advanced on Mel, reaching for her throat, but the Doctor stuck a pair of pliers into Simon's head, short-circuiting him.