Is Robin Ellacott in love with Cormoran Strike? “I definitely don’t want to be,” says Robin (Holliday Grainger) of her friend and private-investigator business partner. “That’d screw everything up.” In the sixth season of Strike, a crime drama based on the sixth Strike novel by Robert Galbraith, AKA JK Rowling, it feels more than ever as though the show itself is afraid that this is true. Grainger and Tom Burke, who plays the titular Strike, do a lovely line in will-they-won’t-they romance, but the series might expire if that ever resolved itself, because the sleuthing part of it is moribund already.

As a work of crime fiction, the TV version of Strike is several removes from having a compelling reason to exist. It is based on a series of hit novels – fine, nearly all detective dramas are – but these books aren’t especially well regarded. They surely attract as many sales and comment pieces as they do only because their author created Harry Potter. That is no help for a TV show trying to succeed on its own terms.

The novel The Ink Black Heart presented the regular Strike screenwriter Tom Edge with his toughest challenge yet. Criticisms of the book coalesced around two observations: at more than 1,000 pages, it was far too long; and its exploration of online fandom and comment culture, often rendered as long transcripts of chat threads and social media conversations, was self-indulgent and impenetrable. Edge has done a decent job of planing that doorstopper into four hours of telly, although his quest to extract a strong narrative from the verbiage was an impossible one.

The case begins when the writer of a cult YouTube cartoon, called The Ink Black Heart, is murdered. The dead author had, in a meeting with Robin, alleged that the anonymous creator of an unofficial spin-off game had been harassing and threatening her. As they line up the chief suspects for the online stalking and subsequent killing, in search of an agitator known only by their internet alias “Anomie”, Robin and Strike consider whether a jealous rival, a spurned associate or the members of a far-right activist group are to blame.

The screen version dispenses with most of the online discourse of the novel – it was almost unreadable, so it would definitely be unfilmable – although there are some lifeless scenes of Robin and Strike scrolling through online comments or trawling video content for clues. (Their gambit is to eliminate suspects by putting them under surveillance and witnessing them otherwise engaged when Anomie is active online.) The unavoidable plot strand where Robin logs in as a player of Anomie’s game descends into the unintentionally hilarious; when Robin’s avatar probes the killer’s cloaked alter ego for clues, their interactions are rendered via crude animation and amusing text-to-speech computer voices.

With the book’s excesses and provocative subtexts stripped away, we are left with a workaday whodunnit in which a small community of people is entirely populated by potentially vengeful oddballs, only one of whom is a murderer. Four episodes later, the revelation is shrug-worthy, even to those viewers who didn’t guess it fairly early on. But Edge hits all the right beats, albeit at the slow pace dictated by the brand commanding a luxurious amount of screen time. It’s not a sufficiently gripping tale for festive BBC One primetime, but it’s not a mess, either.

The real business happens between Burke and Grainger, colleagues who adore each other but cannot say it. This time around, in the opening scene, Strike more or less does say it, when their chemistry becomes undeniable during Robin’s birthday dinner, but Robin panics and pulls away, to her mentor’s embarrassment. They avoid the subject, and each other, dating other people on the incorrect assumption that their beloved has become indifferent. Then the investigation pushes them closer together again and the cycle restarts.

Edge was responsible for the gorgeous romcom Lovesick, so he knows what he is doing here, as do the leads: Grainger’s open-hearted energy, ranged against Burke’s blend of imperious cunning and self-lacerating vulnerability, is affecting. But in the absence of any interesting crime-solving to distract us, the actors’ excellence doesn’t transcend the frustration of their characters’ relationship not being allowed to progress. Like everything else in Strike, the detectives’ love is starting to go stale.

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