Unforgotten season 4 review: The beloved cold case drama is back - with a cliffhanger twist

“You ready?” he asks his colleague - but he may as well be addressing the viewers at home. Sunny and his practical backpack are reunited, which can only mean one thing: Unforgotten is finally back, and there’s another cold case murder to solve. All's right with the world.

But a mournful glance directed to an empty office chair shows us that not everything is quite right in Sunny’s world: his beloved boss, DCI Cassie Stuart (played by Nicola Walker), is on leave and looking to retire early from the force.

Of course, life carries on, and it’s off to a North London scrapyard, where a headless, handless corpse has been found (an early reminder not to pair Unforgotten viewing with your evening meal… ). Sunny and colleague Fran deduce that the long-frozen body had fallen out of a freezer at the site, and that the actual death may have occurred decades ago. A distinctive football tattoo gives them a lead on the victim’s identity.

Meanwhile Cassie is bracing herself for a difficult meeting. She’s requested early retirement from the police following 29 years and nine months of service - just three months shy of the obligatory 30 years.

Her boss is sympathetic, but his hands are tied. He explains in their meeting that if she quits now, she’ll lose a huge chunk of her pension. Cassie’s composure slips - she’s done mopping up the blood and tears, she says. The box-tickers over in Admin have no idea what a job like hers does to a person. But still, there’s no leeway, and she has a difficult decision to make: while she’s desperate to retire, her father Martin has early on-set dementia and may need financial support.

Interspersed between Cassie’s meeting and Sunny’s scrapyard investigation, we briefly meet four individuals who (for anyone who’s never seen Unforgotten) will undoubtedly be our four key criminal suspects. In the Peak District, there’s nervous therapist Fiona Grayson; in Cambridge, engaged career woman Elizabeth Baildon; in Rochester, Dean Barton, a businessman with a rather dodgy past; and in Southall, expectant father Ram Sidhu.

As ever, there’s nothing that seems to connect them - at least initially.

Back at the station, Cassie has returned, despite her best efforts - although she’s delightfully miffed when her colleagues only nod in recognition as she walks in (Sunny, bless him, had told them all to play it cool so as not to embarrass her).

But there’s been a breakthrough in the case: they’ve identified their victim as Matthew Walsh, missing for the past 30 years. And they have a lead: the freezer where the body was kept was previously purchased by a recently deceased man, Robert Fogerty.

The team pull up Forgerty’s records and learn he was caught drink driving the very same night that the victim disappeared. They also learn that there were four extra passengers in Fogerty's car that night.

Dream-team Cassie and Sunny head to a pub to meet the retired officer who wrote up the drink driving report, and spookily, he remembers the incident. Fogerty had been heartbroken when he’d been pulled over because he was a “newly qualified copper,” and a DUI meant he’d just sabotaged his entire career. Not only that, but all four passengers in the car were also brand new officers on their way from a passing out party.

That means the four suspects we’ve just met - Fiona, Elizabeth, Dean, and Ram - were all once coppers, meaning that for our longtime police detectives, Cassie and Sunny, this case just got very personal indeed.

It’s a good twist, particularly following Cassie’s impassioned, angry speech about the police force during her retirement meeting. Series creator Chris Lang has already teased that season four will “ask a number of difficult questions about the police force as an institution” - and from the look of this first episode, it won’t hold back.

Unforgotten continues on Mondays at 9pm on ITV. Looking for something to watch? Visit our TV Guide.