When True Detective (Sky Atlantic) first appeared in early 2014, it was impossible to predict that it would become the cultural phenomenon it did, even with the star power of Matthew McConaughey and Woody Harrelson firing it up from its core. Neither could anyone have anticipated the Icarus-like fall that came with its second season, which took its wares to California and disintegrated into a confusing and aimless mess that wasted the presence of Rachel McAdams and Colin Farrell.

It is little wonder, then, that the third season, which opens with a double bill, has a sense of back-to-basics about it. There’s a hardboiled detective duo at the centre once again, Roland West and Wayne Hays, this time played by Stephen Dorff and an outstanding Mahershala Ali. They’re in Arkansas, investigating the disappearance of two children on bikes, made all the more ominous by the presence of creepy, hand-crafted dolls (a less rustic version of the twiggy effigies in season one, as if the maker had done an NVQ in arts and crafts). It’s a tough, grizzled man’s world, and Ali and Dorff have the gruff deep-and-meaningful mumblings to prove it. It’s as if Rust Cohle never left.

Season one had two timelines; season three has upped the ante to three. The initial case of the missing Purcell children takes place in 1980; Hays appears again in 1990, giving a deposition amid suggestions that the wrong culprit went away for whatever slowly unfolding crimes were committed; and once more in 2015, as an old man with dementia, scribbling his memories down, Memento-style, and giving an interview about the events of the past four decades to a television crew for a show called, wait for it, True Criminal.

This requires a significant amount of chutzpah, from the decision to have Ali play a man at ages 35, 45 and 70 – brave in the age of 4K and HD, although effective, as the prosthetics are incredibly good – to the certainty that this excruciatingly slow drip-feed of information is intriguing enough to span almost half a century. It pays off, to an extent. True Detective has always been a good-looking show, even in its second season, after the distinctive director Cary Fukunaga had left (he turned his TV eye to the visually arresting, if not entirely compelling, Maniac, for Netflix). It revels in building a gloomy, misanthropic world, and its atmosphere is stifling. The cuts between timelines are gorgeous and dreamlike. Naturally, everyone is a suspect, although it does pluck its candidates from a handbook of familiar tropes, from the grieving dad to the drinking, smoking, heavy-metal-loving teenagers and the paedophile who has managed to inveigle his way into volunteering at a local nursery, until Hays and West get their hands on him.

It’s impossible to overstate the value of Ali here, making it plain that the Oscar he won for Moonlight is unlikely to be a one-off; he is remarkable in each of the eras (he also does a great impression of a lightsaber). In interviews, Ali has said that he was initially offered a supporting role, but persuaded the creator, Nic Pizzolatto, to make Hays the lead. The decision to rework the story to that effect clearly benefits all, and it certainly adds depth.

Despite its familiar strengths, though, True Detective falls into bad habits. By the end of the second episode, the pace has begun to slacken, and it is not exactly snappy to begin with. There is a pressing air of self-importance. There is little hope for anyone with an aversion to mumbling. The young female director of True Criminal (Sarah Gadon, who was excellent in Alias Grace) is a millennial cliche who says such things as: “I’m interested in the intersectionality of marginalised groups within authoritarian and systemic racist structures,” a line that seems designed for a laugh, though if so, it’s a cheap one.

In the end, this is a decent, serviceable thriller, nasty enough to give the impression of not flinching away from the darkest natures of its characters, and sufficiently gripping to ensure that viewers should keep on watching, to find out who they fingered for the crimes of 1980, and why this particular case returns to haunt Hays, decades on, like so many TV detectives before him.