Episode 1

The fifth season opens the way a season should not: with bombs. A series of attacks on British targets sends the country into genuine panic, and Parliament responds by rushing through emergency measures that would have been unthinkable a month earlier. MI5's job, in the middle of all this, is to work out who is really behind it. Harry's fear is not the bombers. It is the possibility that influential establishment figures are using the attacks as cover for a coup.

The episode is built on that suspicion, and it gives the team no comfort. Every new power the state grants itself looks, from where Harry sits, like another rung on somebody else's ladder. The visible enemy is the one everyone can see. The man who wrote the law is the dangerous one. The team works the case while half-watching its own government, which is a new kind of tired.

It is a brave way to begin a run, making your own government the suspect, and the show commits to it completely. The attacks are handled with restraint, all aftermath and alarm rather than spectacle, and the tone stays cold where a lesser series would have gone lurid. The season is asking a question it will spend the next few episodes answering, and the answer will be expensive.

Ben Richards and David Wolstencroft wrote it, Omar Madha directed. Peter Firth's Harry leads the fight, with Rupert Penry-Jones's Adam, Lindsay Duncan's Angela Wells, Nicola Walker's Ruth, Raza Jaffrey's Zaf, Hugh Simon's Malcolm Wynn-Jones, Rory MacGregor's Colin Wells, James Dicker's Wesley Carter, Grant Parsons's Surgeon and Howard Teale's Politician filling the hour.

But the real story is the one the team cannot quite see yet. The coup, if it is coming, is coming politely, in suits and signatures. That is what makes the opening hour so unsettling, and so worth watching.