An ambulance drives through Sydney, having been hijacked by escaped convict Matt Kirk and his three accomplices, Matt's brother Johnny, Italian Luke and Bert. The four men manage to avoid detection at Sydney Hospital, and head out through Sydney Harbour in a purchased vessel, intending to go north. However the boat breaks down before they can get through Sydney Heads and the men decide to take refuge in Fort Denison (also known as Pinchgut), unaware it is occupied by caretaker Pat Fulton, his wife and daughter Ann.