Unforgotten, Series 2, ITV

Historic unsolved murders have become their own mini-genre, with the likes of Cold Case lurking in the small print of the schedules and Silent Witness still going strong in its 20th series. A hit the first time out in 2015, Unforgotten is back with a new investigation of another mystery cadaver.

This time, the deceased was dredged out of the River Lea in north-east London, having been crammed into a suitcase (post-mortem, one hopes). Detectives Cassie Stuart (Nicola Walker) and Sunny Khan (Sanjeev Bhaskar) were promptly down on the riverbank, poking at the mysterious plasticky skin of the victim and trying to decide how long it had been dead. The tight-fitting suitcase had successfully kept the water out, so it could have been there for 20 years or more.

Unforgotten doesn't try to bamboozle the viewer with scientific gimmicks or unfeasibly tangled plotting, which may be one of the keys to its success. Instead, the forensic work to establish the victim's identity focused, plausibly enough, on the deceased's watch. The jeweller who repaired it had left his own mark on the inside of the case, so it was a merely a matter of bashing the phones and pounding the pavement until the jeweller in question was located. He in turn had a record of his customers, and bada-bing – the corpse had a name (David Walker) and before long all the baggage of his former citizenhood began to accumulate around him.

But where will it all lead? This first episode obligingly identified the victim and also introduced a list of characters whose interrelationships are, so far, unknown to us. We have nurse Marion (Rosie Cavaliero), who seems to have a too-close relationship with her patients and has fallen out with her sister. Down in Brighton, there's lawyer Colin Osborne (Mark Bonnar, pictured above), who's half of a gay couple with an adopted young daughter, Flo. Having responded, rashly perhaps, to some vicious homophobic abuse from a man in a supermarket car-park by dragging a key down the side of his Range Rover, Osborne was aghast to find this had left him open to blackmail by a sleazy-looking character called Tyler. He claims he's the partner of Flo's birth mother.

Then there's Salisbury-based schoolteacher Sara Mahmoud (Badria Timimi, pictured left), who we first met as she set her pupils the task of deciding whether Lady Macbeth was more evil than her husband. A provocative question indeed. Then Sara went for a job interview for a head-teacher's job at another school, pressing her case with an impassioned plea to her interrogators that she would never give up on any child.

So far, so inconclusive, though we do know the crucial fact that about-to-retire Detective Inspector Inspector Tessa Nixon (Lorraine Ashbourne) is the dead man's widow, now remarried. And the discovery of the remains of a pager inside the murdered man's chest – his killer stuck a knife through it – is about to yield some secrets. But what makes Unforgotten tick is the low-key, underplayed relationship between Cassie and Sunny, who are both adept at evoking characters who are committed to their work while doggedly coping with the routine demands and frustrations of their personal lives. This isn't revolutionary television, but it's believable – mostly – and keeps its feet firmly planted on the ground.

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