Episode 8

Khalid Mansour should have been put away on the strength of MI5's video surveillance. Instead the footage is ruled inadmissible, and the suspected terrorist walks free from the courthouse. He does not get far. On the steps, moments after his release, he is shot dead, and the assassin, Daniel Ellis, turns the gun on himself. A second suicide attack takes a radical Muslim cleric, and an anti-Islamic group calling themselves the Sons of Phineas claims responsibility.

MI5 has never heard of the Sons of Phineas, but the trail leads somewhere familiar. Both dead men lived at a homeless shelter run by Steven Paynton, and the team builds a sting to persuade Paynton to name the people he has recruited, hoping he leads them to the organisation before the violence gets worse. With racial tensions climbing across Britain, Adam goes undercover to draw him out. Paynton, the episode suggests, is a radicalised Christian determined to incite a religious war.

The name the group chose tells you most of what you need to know. Phineas, the biblical figure from the Book of Numbers, was the man who stopped a plague with a spear, and his modern namesakes read the same text and reached for weapons. The episode handles the material with more restraint than that sentence suggests, and Shaun Dingwall's Paynton is a quietly chilling portrait of conviction.

Julian Simpson and David Wolstencroft wrote it, Julian Simpson directed. Peter Firth's Harry, Rupert Penry-Jones's Adam, Hermione Norris's Ros, Raza Jaffrey's Zaf, Lennie James's David Newman, Gugu Mbatha-Raw's Jenny, Matthew Blackmore's Poe, James Dicker's Wesley Carter and Rogan Grant's Homeless Militant complete the cast.

And then the job turns personal. Adam's young son disappears, and the episode stops being about ideologies and becomes about a father whose work has followed him home. It is a cruel, effective reminder of what the job costs, and it lands.