Episode 1

Series three opens with the news MI5 never wants: Tom Quinn has shot Harry Pearce and vanished. As if that weren't enough, he also stands accused of assassinating the Chief of the Defence Staff. The service that usually does the investigating is suddenly the one under investigation. A service that can't account for its own people is a service that can be taken apart, and Oliver Mace intends to do the taking.

Mace, chairman of the Joint Intelligence Committee, sees his opening. He launches a punitive inquiry into MI5 that threatens to swallow the whole organisation. Zoe and Danny are left to prove Tom innocent while the government circles overhead. And every hour without answers makes the case for a takeover stronger. The pressure isn't just professional. Tom was one of them, and the service has to decide whether to defend a colleague who may have gone bad.

Matthew Macfadyen plays the missing Tom in flashbacks and absences, and Keeley Hawes's Zoe carries much of the legwork. David Oyelowo's Danny, Tomas Arana's Herman Joyce, Michael Culver's Hugo Weatherby, Megan Dodds's Christine Dale, Shauna Macdonald's Sam Buxton and Rory MacGregor's Colin Wells make up the team around them. Tristan Hickey turns up as a CIA man, Fiona Carew as a nurse. Peter Firth is Harry, present mostly as a problem to be solved. Howard Brenton and David Wolstencroft wrote it; Jonny Campbell directed.

It's an angry, unsettled start to the series, built on the idea that the institution can tear itself apart faster than any enemy could. For anyone who has followed Tom since the beginning, it lands a particular kind of blow. What it leaves behind is a sharp question about whether the service can survive finding out the truth about one of its own. It's not comfortable viewing, and it shouldn't be.