Tyssan was a starship engineer who served with Earth's deep space fleet. He was captured by the Daleks and imprisoned on one of their spaceships until fifty slaves including himself were taken down to Skaro to clear out a mining area where the Daleks had been excavating. Tyssan managed to escape two years after his capture.
He (and all the other slaves) were informed by the Daleks that every time a slave escaped, five others would be exterminated in their place. Tyssan avoided recapture and, while exploring Skaro, startled Romana II, whom he had watched, after her and the Fourth Doctor's arrival on Skaro.
After the Doctor bargained for the release of the slaves from the Daleks, Tyssan led their attack on the Movellans' ship. After their victory, Tyssan led a charge against a group of Daleks sent to destroy the Movellan ship they had taken over. Tyssan remained in charge of the Movellans' craft following the second attack and assisted in transporting a cryogenically frozen Davros back to human space. (TV: Destiny of the Daleks)