A strong premise lets a crime drama like Unforgotten not just survive but thrive on casting changes

Fortunately, he’s an off-duty TV detective, which means he’ll be saved from this existential crisis by his phone, calling him away to some freshly discovered bones in a marsh.

Sanjeev Bhaskar’s DI Sunil 'Sunny' Khan almost skips out of his seat to attend this grisly scene. Meanwhile, also in need of rescuing is his boss DCI Jess James (Sinéad Keenan), busy wondering why her husband has cooked her a surprise seabass for dinner. Anything to do with the apparently female hair she’s discovered on his coat lapel? Before the seabass takes a flier, her phone rings and she too is bog-bound.

Two cops, two troubled personal lives in the welcome return of Unforgotten, where solving the murders of long-dead corpses is much more straightforward than dealing with everyday challenges.

In just their second season together, Sunny and Jess rub along cosily like two weathered slippers, remarkable when you think of fans’ devastation at the abrupt, tragic departure of Sunny’s former boss Cassie (Nicola Walker).

So close were this erstwhile duo, they were in danger of falling into the Moonlighting trap. With such palpable chemistry, how did writer Chris Lang paper so effortlessly over the join?

The answer is, he leant into the grief. Throughout the last season, Sunny battled with resistance to Cassie’s replacement before gradually beginning to repair, as did we. Lang even gave Jess a line that could have been Keenan speaking: "I’m aware of the boots I’m filling… I sincerely hope to do her and her team justice."

One short season later, it’s as though Jess was always there. We viewers are a fickle bunch.

Unforgotten is one more series proving that the disappearance of the big name above the title needn’t mean the show’s demise, with crime drama a particularly robust genre.

Douglas Henshall was the undisputed star of Shetland, yet it hasn’t skipped a beat with Ashley Jensen’s arrival in his place. It also lent into the locals’ discomfort, fidgeting around newcomer DI Ruth Calder until she’d earnt her tartan.

And, with stalwart Alison O’Donnell as Tosh, the show has enjoyed a fresh dimension, the two women supporting each other through messy personal lives carefully not discussed over big glasses of wine, a Caledonian Cagney and Lacey, no less.

The sudden departure of Death in Paradise’s first detective, Richard Poole, inspired grief-wracked gestures from his Saint Marie acolytes back in 2014. But this franchise has ventured into Doctor Who territory with its changing of the guard, and after 14 seasons and five DIs, the cast have given up pretending to look surprised.

Meanwhile, in Midsomer Murders, Neil Dudgeon has been the resident DCI Barnaby for as long as his predecessor John Nettles. Of course, the ultimate proof of the thesis is Taggart, a show so established in tone and setting that it withstood even the loss of the name on the tin.

Last week, I wrote about how comedy spin-offs require a strong lead character to hold our interest, but crime drama is the opposite. If the premise is strong, it can not only withstand but flourish from casting upheavals.

Each season of Unforgotten explores disparate present-day lives connected to a past full of love and loss. Sunny’s shift from Cassie to Jess is entirely in keeping, a fitting nod to the passage of time in all our lives.

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Unforgotten season 6 will premiere on ITV1 and ITVX on Sunday 9th February at 9pm.

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