Face of the Enemy
Featuring: , Brigadier Lethbridge-Stewart, Sergeant Benton, Mike Yates, Ian Chesterton, Barbara Wright, Harry Sullivan
A few days after the TARDIS leaves there is a strange plane crash and the body of the pilot almost exactly matches a junior minister's. UNIT is called in and the Brigadier drafts Ian Chesterton from the Air Force as a temporary Scientific Advisor. When UNIT call on the minister in question they discover that he has been murdered. The following day, the Master escapes from Aylesbury Detention Centre. He arrives at UNIT HQ to give himself up in return for protective custody. He helps UNIT build a device to track the duplicates, who are using some kind of time travel technology. Meanwhile, Barbara Chesterton is kidnapped from UNIT HQ and apparently killed in a car crash.
The Master and Ian manage to track some temporal disturbances to the Naval base at Faslane, which seems to be the centre of the infiltrators' activities. UNIT staff travel to the base, where they meet Surgeon-Lieutenant Harry Sullivan. Sullivan helps them find out where on the base the infiltrators have been hiding. The Brigadier, Ian, and the Master travel to the Inferno universe, where they rescue Barbara and disable the infiltrators' ability to travel between worlds. They then stop the infiltrators from getting away in a nuclear submarine and recapture the Master.
The Doctor and Jo return to Earth two weeks (Earth time) after they left and the Brigadier doesn't tell them what has happened since they left.