Undertow (1)

A man is due out of prison. On paper he is a non-violent offender, served his time, entitled to walk. The team is convinced he is a serial killer responsible for a series of attacks and murders of women. Undertow (1) is the race to prove it before the cell door opens for good.

That is a good engine for an hour of television, because the system is not on their side. Suspicion counts for nothing against a release date, and the squad has to turn certainty into evidence while the calendar works against them. Boyd hates this kind of case. It asks him to be right and fast at once, with Grace Foley's read of the psychology pulling one way and what can actually stand up in a room pulling the other.

Oliver Brown and Barbara Machin wrote it, David Thacker directed. Trevor Eve leads as Detective Superintendent Peter Boyd, with Sue Johnston as Grace, Wil Johnson as DI Spencer Jordan, Esther Hall as Dr Felix Gibson and Félicité Du Jeu as DC Stella Goodman. The guest cast gives the family around the case real weight: Stephen Moyer as Steven Hunt, Cheryl Campbell as Maureen Hunt, Peter Wight as Donald Hunt, Dan Fredenburgh as Ben Elwes and Sharon Duncan-Brewster as Sarah Baker.

The show has always been good at this particular frustration: the gap between knowing something and being able to say it in a courtroom. Part one leans into it. Every lead the team chases raises the same question of whether it will arrive too late to matter.

And the episode is honest enough to leave open the possibility that they are wrong about him entirely. That doubt is what keeps it honest: the team has been certain before, and certainty without proof is exactly how innocent men end up inside. Boyd has to hold his conviction lightly while acting on it completely, which is a hard thing to ask of anyone.

As a first half it does its job well. You leave it hoping they are right, and slightly afraid to find out.