Deep Waters

Frost finally has a home again — and, in short order, a pair of reformed criminal lodgers. The domestic comedy is a brief mercy before the case arrives. Helen Tudor, a beautiful, sweet-natured psychology student at Denton University, is pushed down a staircase after a private tutoring session, her only memory a voice begging her not to die there; suspicion first falls on her tutor, Dr Keith Michaelson, a lecherous academic who exploits his standing to bed students and was rebuffed by Helen. Then on her older boyfriend, who turns out to have a pregnant wife. But the case opens sideways when DS Clive Barnard, now based in Boxborough, flags a similar victim: Paula Tate, a keen swimmer who looked like Helen, drowned two years earlier in what was ruled a suicide. Frost and WPC Lindsey Hunter, his bright young assistant, begin circling Helen's fellow student Adam Weston, who knew Paula — and then the poem arrives, describing Helen as a flawless mermaid.

The truth is waiting in Weston's orbit. His friend Tony Jarvis, who sneaks into the university pool to swim naked, had spent a fortnight studying in Boxborough — the same town where Paula died. Jarvis's mother, confronted, admits what she has hidden for years: as a child, Jarvis drowned his own younger sister, Simone, in a jealous rage, and she covered it up to keep her remaining child. Frost reaches the scene in the nick of time to stop Jarvis from drowning Helen. In interrogation, Jarvis explains himself with chilling lucidity: he believed Simone and Paula were beautiful mermaids he was freeing to swim forever, and he attacked Helen because he needed to make sure Michaelson had not spoiled her first.

The second case is a wound left open. A post office robbery kills the owner's wife, and Frost arrests the Bainbridge cousins — but without witnesses, he must release them, vowing to find evidence even if it takes years. Deep Waters, written by Christopher Russell and directed by Don Leaver, is the series' darkest episode yet: a killer whose madness is a kind of love, and a detective who cannot close every case no matter how hard he tries. Damian Lewis, early in his career, makes Adam Weston's guilt shimmer just below the surface.

A dark, melancholy hour with a killer who drowns his victims out of a twisted love, and a detective who must live with the cases he cannot close. Damian Lewis, early in his career, makes his mark. The darkest episode yet, with a killer of terrible gentleness.