Unforgotten: Sinéad Keenan’s fantastic as the new Nicola Walker. What’s with the accents, though?

It’s a long way from Fair City to the London forensic labs that are the natural habitat of the flinty detective played by Sinéad Keenan in Unforgotten (UTV, Monday, 9pm). Yet that’s the journey the Dublin actor has completed as she takes over from Nicola Walker as the lead for season five of Chris Lang’s agreeably pulpy cold-case thriller.

Walker’s exit from Unforgotten nearly burned down the internet two years ago. Her character, the wonderfully deadpan Cassie Stuart, died in a car crash after solving her latest investigation with the help of her calm-and-steady number two, Det Insp Sunny Khan (Sanjeev Bhaskar).

Stuart’s departure was such a surprise that Keenan initially considered turning down the part. “I said, ‘Thank you very much, no ... Who’s going to be the gobsh*** to follow Nicola Walker?’” But she was talked around – and is so compelling as Det Chief Insp Jessica James that it takes only a few minutes to forget that Unforgotten was once The Cassie Stuart Show.

Not that the grisliness is any less pronounced. The conceit of Unforgotten is that, when it comes to violent crime, all grieving families deserve to know what happened to their loved ones, regardless of whether they were killed 12 months or 40 years ago. In this latest season, the body of a young woman is found in a chimney flue of an Edwardian house in Hammersmith, in west London. The discovery is made by thumping coincidence on the very morning that James takes over from Stuart.

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The day turns out to be weird in more ways than one for the DCI: she’s about to leave for work when her husband causally mentions that he’s been having an affair and their marriage is therefore on the rocks. James arrives at the office in an understandably stinky mood and quickly gets down to business – that of not getting on with Sunny.

He’s still holding a torch for Cassie, who believed completely in the mission of the cold-case division. James, by contrast, believes police resources could be better spent. When initial lab reports suggest the woman in the chimney was killed 50 years ago, she calls off the investigation. But Sunny goes behind her back and establishes that the vintage dress the victim was wearing was bought just six years ago. This cold case isn’t terribly cold.

The strangers whose stories will intersect with the investigation turn out to include a political grandee played by the Northern Ireland actor Ian McElhinney and his wife, played by the former Pollyanna Hayley Mills.

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Keenan is great as the outwardly tough, inwardly vulnerable James. Yet, as is often the way with Irish protagonists in British drama, the show doesn’t do a good job of explaining how an Irishwoman ended up as a senior Metropolitan Police detective. She even has her mother over with her. And she, too, has an Irish accent (sort of – it’s an English actor trying a low-key brogue). How they both got there is entirely unclear. It’s one mystery this otherwise excellent drama seems ill-disposed to solve.