Kirksen was a criminal stranded on Desperus.
Kirksen boarded the Spar when it was forced to land on Desperus. He took Katarina hostage and tried to blackmail the First Doctor into taking the ship to Kembel. Bret Vyon refused to obey the criminal, while Steven Taylor wanted to save his friend. The two argued over what to do until Katarina rendered the argument moot when she opened the airlock door, killing Kirksen and sacrificing herself. (TV: The Daleks' Master Plan)