Cross (Frontier in Space)

Cross was a non-political prisoner at the Lunar Penal Colony who was looked on with favour by the prison governor. Because of this, he was the leader of a section of the prisoners. To them, he was the effective "voice" of the Governor. He performed many of the functions of a lead guard, despite the fact that he was a prisoner himself. At one point, he pretended to help the Third Doctor and Professor Dale escape, then tried to kill them by depressurising the airlock they were in. (TV: Frontier in Space)