Writing last month about the superb Netflix version of House of Cards, I noted in passing (and with regret) how the first episode of ITV's 8-part drama Broadchurch had plonkingly reminded us about the possible suspects in the mystery of who had murdered a 12-year-old boy. Trust the audience more, I urged the writer: perhaps I should have trusted him more. 

Broadchurch sustained a large audience throughout its run, peaking with this week's 9 million viewers for the final episode. As it turns out, the question of whodunnit was clearly not the main attraction: most viewers (judging by the tight odds on the actual killer offered by bookmakers in the run-up to the denouement) had worked this out. Indeed, the culprit confessed before episode 8's first commercial break. 

In many ways, Broadchurch broke basic rules for murder serials. The writer, Chris Chibnall, was clearly in love with his location (a Dorset seaside resort, boasting a sandy beach backed by dramatic cliff formations). He was determined to explore the inner workings of a small community: Twin Peaks was one of his conscious models - another was Murder One, which set the bar, a dozen years before The Killing, for extended exploration of a single crime. 

In a small town, all kinds of plot improbabilities are tolerated. The local police sergeant, Ellie Miller (played with characteristic down-to-earthness by Olivia Coleman), misses out on her expected promotion from sergeant, which instead goes to a scruffy, bearded outsider, Alec Hardy, played with a strong Scots accent by David Tennant. We later learn he was not really an outsider - he had holidayed at the resort when he was a child, and remembers sitting on the beach looking out to sea (quite what a Scots family was doing taking holidays in Dorset is unexplored).

Once the body of the victim, Danny, is found lying on the beach the morning after Hardy takes up his post as inspector, he is on a mission of redemption, burdened as he is by the reputation for having recently failed to solve a similar child murder. He has also concealed a serious medical condition - heart arrhythmia - from the police authority, so when he collapses, twice, and is hospitalized, he is told (somewhat illogically) that he has just one more day to wrap up his lengthy inquiry.

By then, he has belatedly recovered the hard drive of the laptop belonging to Danny's best friend - Ellie's son, Tom - which should have been an early target of his investigation, and which - as viewers of the first episode had clearly understood - contained the key to the case. Tom had fallen out with Danny just before the murder: Danny had a new friend - who turns out to be (as we suspected) Tom's dad, Joe. Just as well Ellie didn't get that promotion! And a pity she never asked her son for any emails he might have kept from Danny. 

We finally discover that Joe "loved" Danny. He had not yet got round to molesting him, but had given him £500 in cash, which had puzzlingly turned up when Danny's room was searched. It was apparently an unspent part of the Miller family's spending money for their recent Florida holiday. Taking a sleeping pill to get over jetlag was why Ellie failed to notice her husband's prolonged night-time absence from their bed on the night of the murder (meeting Danny, strangling him in panic when Danny threatened to tell all, carrying the body, plus Danny's skateboard, to a rowboat, rowing the body and skateboard to the beach, leaving both body and skateboard there, and elaborately cleaning the crime scene whilst preserving Danny's incriminating mobile phone). Not realising that she was missing £500 in sterling notes unspent in Florida suggests Ellie's powers of observation were seriously flawed! 

Before the end, Danny's mum confronts her erstwhile friend, Ellie: "how could you not know?", echoing Ellie's own disbelieving words to another character, Susan, who claims not to have realised that her husband had been abusing one of their daughters.

Susan has turned up in the resort to contact her son, whom she has traced to Broadchurch. He had been taken away from her in infancy, after her husband was jailed and the abused daughter had committed suicide. She confuses the police by claiming that she had seen this son - who has rejected her - on the beach with Danny's body (her "son" is a good six inches taller than the killer, but she has concluded that he carries the bad genes of his abusive father). She also confuses everyone, not least the viewing millions, by examining the body, smoking a couple of cigarettes whilst doing so, choosing not to report her findings, and making off with Danny's skateboard (which she later, even more bemusingly, gives to Tom).

As if these McGuffins, red herrings and utter improbabilities were not enough, Chibnall also gives us another possible paedophile suspect, the local newsagent for whom Danny used to do a paper round. He turns out to have a conviction for sex with an underage girl (a 15-year-old whom he married immediately after serving his 12 month sentence). Because he also organises the local brigade of teenage sea scouts, he becomes the target of a hate campaign, led by Susan's son, and fuelled by a local journalist (Ellie's nephew!), who has discovered his secret past. He drowns himself in the sea.

Two other "suspects" we are offered are Danny's father, whose nocturnal absences seem not to register with his wife (actually, he is boffing the attractive manager of the main hotel); and the young vicar of the Anglican church, who happens to double as IT tutor for both Tom and Danny. We also have the local clairvoyant/nutter, receiving messages from dead Danny ("it was someone he knew", "he was in a boat") as well as unsettling Hardy with his tantalising glimpses from the past.

If these plot points lack plausibility in naturalistic terms, there is yet more to be revealed. When the local journalist confronts Hardy about his hospitalization, Hardy bizarrely offers to explain his previous murder inquiry's failure in exchange for not having his illness publicised.

The explanation stretches credulity even further: apparently, Hardy's wife had been working on the case as a detective sergeant, and having an affair with another detective sergeant, and had stopped off for a drink with him whilst leaving the crucial evidence and her handbag in her car, both of which had been stolen when a thief broke in.  Even more incredibly, the journalist agrees not to reveal any of this, for fear of the harm it might inflict on Hardy's family. 

It is the very absurdity of this narrative that should alert us to what Chibnall is actually on about: how can any of us know - really know - what our neighbours and even our loved ones are like, even in a small community where everyone in theory knows everyone? 

The expected quota of naturalism in the production is leavened throughout by slow motion shots, static landscapes of sometimes startling beauty, and a remarkable sense of place. Yet the intimacy, however beguiling, is deceptive. Outsiders like the newsagent and Susan, seeking refuge from troubled pasts, are forced to leave, whether by suicide or midnight flit. Treachery and deceit are basic human traits, even within the most close-knit relationships. 

And Chibnall, surprisingly, pulls one more rabbit out of the hat before the end. In yet another church service, the vicar calls on his congregation to learn forgiveness, as God showed forgiveness to mankind through Christ. This startlingly overt religious message is then amplified by the final scenes, where a series of bonfires and beacons in memory of Danny are lit, in co-ordinated fashion, across the Dorset coastline. Danny's mother has forgiven her husband (she is expecting a baby), and with the rest of her family lights the first beacon; the hotel floozy is allowed to light a bonfire; the clairvoyant looks on approvingly; Susan's "son", the lynch-mob leader, joins the ceremony. Are these proceedings pagan? Or Christian? Whatever, the vicar presides.

Meanwhile, Ellie is reconciled with her errant older sister, and her nephew - the dodgy journalist - brings toys for her children to the hotel where she has taken refuge whilst her house is searched for clues (she unsubtly crunches a slug underfoot as she leaves her home). Even the nephew's dodgier colleague, from a national title, where she specializes in child murder, is allowed back into the circle of forgiveness. Hardy - redeemed on his last day - is reconciled with Ellie, who had bitten her tongue almost all the way through in frustration at his working methods. 

Redemption and salvation in an ITV murder mystery? Feel-good Christianity in a drama about paedophiliac killers? Perhaps that explains the mass audience - or perhaps the mass audience barely noticed the religious dimension to what might otherwise have been classified as just another Monday night series. Perhaps the title should have forewarned us. Either way, a return visit to Broadchurch is promised for next year.