Episode 1

The season opens with Tom Quinn's family trapped in his own house with a bomb in it. There is nothing anyone can do, and the credits roll over the explosion. It's a brutal way to start a series. But the show doesn't hang around to mourn: a Serbian warlord has been seen entering the UK, vowing revenge for his sons, who died in a NATO bombing raid. MI5 thinks a revenge campaign by a Bosnian war criminal is underway, and Zoe goes undercover to find him before anyone else dies.

The bomb business hangs over the episode like a shadow. Tom has to keep working while part of him is still back in that house, and Matthew Macfadyen plays the split well. The warlord plot gives the hour its engine, and the supporting cast gives it texture: Bill Buckhurst's Bomb Disposal Officer, Martin Cole's SAS Officer, and Benedict Cumberbatch, a few years away from global fame, as the mysterious Jim North. Even in a small part he brings the watchful stillness that would later make him famous. The episode saves its big effects for the bomb and lets the acting do the quieter work.

David Wolstencroft wrote it, Bharat Nalluri directed. Peter Firth's Harry keeps his usual counsel, Keeley Hawes's Zoe carries the undercover thread, and Lorcan Cranitch's Patrick McCann and Enzo Cilenti's Carlo Franceschini keep the series' familiar faces in view. Rebecca Johnson's Trendy Civil Servant and Emma Barton's MI5 Officer 2 round out a busy week. Teresa Churcher's Sarah and Graham Christopher's George fill out the edges, and the episode doesn't waste any of them.

It's a strong opening episode: personal loss and international revenge in the same hour, with the show's usual refusal to look away. The explosion is the hook. What Tom does with the fallout is the story.