Geralk

 enjoyed the interconnectedness of the main city of Krelos when he used a dreaming machine. He enjoyed going on expeditions in those dreams. The dreaming machine connected to a mechanical body which helped him in his dreams. He lost concentration and bumped into the Fourth Doctor and Leela. Leela thought that he died by the time she had got back from the TARDIS. (AUDIO: The Fate of Krelos)

When the Doctor travelled back in time to stop the Cyberman invasion, he thought that the Cybermen didn't exist. He was converted into a Cyberman. When time was rewritten, he had a fish supper with the Doctor and Leela. (AUDIO: Return to Telos)