Dethras

 was a Xancari scientist. He was part of a British submarine crew and used the name John. The Fourth Doctor realised that he wasn't part of the crew by the fact that he knew that it was primitive and was scared when he appeared. Dethras had discovered the theory of hyper-evolution and experimented on the humans whom Flague had taken. He realised that this research was wrong so he destroyed it and took his experiments away for them to live in peace and not be used. The Doctor told him that this was futile if Flague ever captured one of his creatures. When the Doctor took the creatures to a primitive world, he stayed behind to help them become human again. (AUDIO: Dethras)