Vera, Series 4, ITV

She drinks beer, drives a Land Rover and can never remember the names of her sidekick’s wife and daughter: welcome to the offbeat world of Vera Stanhope, deliciously imagined by writer Ann Cleeves and actor Brenda Blethyn. ITV’s Sunday night cop show-by-the sea, Vera, is back with a fourth series which will be welcome news for a loyal few million viewers and for the people who like to sell Northumberland as a tourist attraction.

ITV may have forgotten to have given it an interesting title (she could so easily be either a Gwen or a Madge) but in tonight's episode Our Vera was anything but dull. It was the first of four new stories, and based on Cleeves’s latest novel in the series, Harbour Street.

What happened in our past inevitably shapes us into who we are now, and crime drama relies on this mechanism to move a story forward. A large cast of characters is linked to a seaside guest house, giving plenty of scope for complex relationships and huge helpings of back-story involving prostitution and unrequited love.

A mature woman is found stabbed to death on a crowded city train. Earlier, Margaret (Annabel Leventon) has tried to tell the police something important but, in a nicely observed scene, has been left unattended down at the station because the dopey copper due to see her has been doing his IT training.

Whatever her message was, it's enough for someone to seek her out and kill her. Vera and Detective Sergeant Joe Ashworth (David Leon, excellent as ever) are drawn into the world of the guest house where she has permanently lived for years. Its owner Kate (Eva Birthistle) said Margaret was much-loved. Slowly, the people around her present themselves while Vera and Joe, fortified by lunchtime ale and chips, poke their noses into a tightly-knit community. Step forward Kate’s fiancé, an eccentric male guest, a prostitute who, we learn, has "been hurt" in the past by Margaret, an elderly former pub landlady and a strong, silent type who messes about with boats.

While such a rich collection of characters makes playing armchair detective especially pleasing, it also presents challenges: namely that some of the performances are uneven and some of the storytelling simply gets lost in the two-hour timeframe. The thread involving Kate, for example, who doubles up running a guest house with being a well-known recording artist, not only stretchea credibility but also doesn’t go anywhere. And while coincidences do happen, does the person who found Margaret dead on the train really have to be DS Joe’s daughter? It offers a potentially interesting sub-plot when Jessie (Olivia Armstrong, pictured above with Leon) begins having nightmares, but the exploration here feels half-hearted.

More positively, the terrific Lynn Farleigh as a retired publican and Paul Copley, equally good as the boatman, do just enough to suggest lives of bitterness and regret, yet other scenes – specifically in the boarding house ­– feel like they’ve been borrowed from an as-yet unknown collaboration between Alan Ayckbourn and Harold Pinter.

It’s something of a golden period for Cleeves, whose other character, Jimmy Perez, has just finished a run in the BBC's Shetland, on first showing a darker series than Vera. But ITV clearly has a winner here, largely thanks to Blethyn's batty but believable performance. She’s quirky, driven and utterly in control of her police work. So far so good, but whether it can become an all-time favourite in the mould of A Touch of Frost and Inspector Morse remains to be seen.