Mysterious man (The Lazarus Experiment)

The mysterious man was an agent of "Harold Saxon" who oversaw Saxon's good friend, Richard Lazarus's experiment to make himself younger. He persuaded Francine Jones that she should not trust the Tenth Doctor. (TV: The Lazarus Experiment) This led to Francine's decision to let another one of Saxon's agents track the Doctor's whereabouts. (TV: 42)