The Prosecutor was the prosecutor for the Dastrons during their trial. The spacecraft they were travelling in crashed in Dunstanton Lake.
Centuries later, he was reactivated and killed Jeff, one of the archaeologists at the lake. He originally thought the Third Doctor, Jo Grant and Freda Mattingly were trying to free the Dastrons, but soon began to trust them.
It was later destroyed when the spacecraft exploded when the Dastrons tried to take off. (AUDIO: Prisoners of the Lake)