Sam Riley and Stacy Martin in Sun-Blasted Noir

The establishing scenes of Islands feel like something out of a James M. Cain novel — a wily noir, in this case drenched in subtropical heat and blinding sunshine. A tennis coach hiding out from life while working at a Canary Islands luxury hotel falls under the spell of a beautiful married guest who seems oddly familiar, with a douchey husband practically begging to be murdered. The thought that Sam Riley’s Tom might turn out to be Fred MacMurray in Double Indemnity or John Garfield in The Postman Always Rings Twice is tantalizing but gradually appears to be more like teasing misdirection as the movie shifts into psychodrama gear.

German director Jan-Ole Gerster’s first English-language film remains absorbing and has a low-key but effective payoff. It’s carried by compelling performances from Riley and Stacy Martin, by its atmospheric setting and by the reliable pleasures of the troubled vacation subgenre. But at two hours, Islands outstays its welcome, allowing much of the tension to leak out of it in a protracted concluding stretch.

Dissolute Brit Tom spends his days giving tennis lessons to guests on the Fuerteventura hotel courts and his nights getting wasted at local dance club Waikiki, where he hooks up with tourists and sometimes wakes up the following morning in the sand. He generally arrives late for court bookings, looking seedy. You get the impression this is a long-running pattern, in which one day bleeds into the next without much to tell them apart.

If Tom feels the pull of a return to the real world, he doesn’t seem to acknowledge it even to himself. But tremors from the volcano on nearby Lanzarote seem like an ominous signal to move on. The same goes for the camel that keeps escaping from a farm owned by his friends Raik (Ahmed Boulane) and Amina (Fatima Adoum), who are selling up and moving back to Morocco.

But as we observe Tom careening toward burnout, he gets shaken out of his daze by the arrival at the hotel of Anne Maguire (Martin), who schedules tennis lessons for her 7-year-old son Anton (Dylan Torrell), offering to pay double when the coach says he’s booked up. Tom is nagged by the feeling that he’s met Anne before.

Her husband Dave (Jark Farthing) is the kind of slick, aggressively competitive jerk who would be right at home in The White Lotus, and Anne does little to hide her irritation with him. But when Tom intercedes with his friend Maria (Bruna Cusí) on the hotel reception desk to sort out a problem with the family’s suite, they insist on taking him out to dinner.

In an uncharacteristic departure from his routine and from his habit of maintaining distance, Tom offers to take them on a driving tour of the island on his day off. The excursion starts with eerie volcanic caves and continues on the beach, where Dave wanders off vaping while Anne asks Tom to rub sunscreen on her back in what seems a classic femme fatale move.

They have dinner at Raik and Amina’s farm after Anton rides a camel, and the Maguires learn that Tom earned the nickname “Ace” when he outplayed Rafael Nadal on the court, while filling in for the tennis champion’s training partner. A nightcap on the guests’ terrace follows, with Tom growing uncomfortable as the tension in their marriage shows, partly over their struggle to have a second child. Nonetheless, Dave coaxes Tom stick around after Anne goes to bed.

Telling Tom that he envies his freedom and absence of family ties, Dave insists they go for one drink at the Waikiki. But one drink turns into several once Dave, who’s supposedly been sober for years, starts hitting the vodka and disappears onto the packed dancefloor sniffing after some surfer chicks.

Tom wakes up in his usual blitzed state on a lounge chair by the hotel pool the following morning and learns from Anne that Dave never came back to the suite. Their search for him turns up nothing, so they go to Tom’s cop buddy, Jorge (Pep Ambròs), who reports that there’s been no sign of him at the local hospitals. As Dave’s disappearance stretches on, eagle-eyed detective Mazo (Ramiro Blas) steps in, questioning both Anne and Tom like potential suspects and turning up inconsistencies in Anne’s account of the night’s events.

There’s a sly playfulness in Gerster’s handling of the story, molding a character study into something closer to a thriller. He’s helped by Dascha Dauenhauer’s tense score and by the moody visuals of DP Juan Sarmiento G., with events at times seeming to unfold from a hazy distance. The location is used to great effect, injecting unease with elemental noise like the pounding waves of the Atlantic crashing in.

It’s nice to see Riley — who has never gotten a part as interesting as Ian Curtis in the Joy Division bio-drama Control — in a leading role. With his lanky frame and slightly ravaged look, he gives Tom a sad sense of resignation, defeat, exhaustion. But Riley drops enough gentle hints of longing for something more to make it ambiguous in the conclusion as to whether he will snap out of that or revert to it.

He trails along helplessly in the wake of Martin’s beguiling but more inscrutable Anne. He’s disconcerted but not deterred when her behavior becomes less and less like that of a woman whose husband is pretty much presumed dead after his shirt and wallet are found on the rocks overlooking a patch of ocean known for its treacherous currents. Both actors effectively convey the slow-burn attraction between their characters.

Gerster keeps a twist or two up his sleeve but he’s too deliberate about revealing them, overdoing the foreshadowing to a point where Tom’s big realization after spending time with Anne and Anton comes as no surprise. Still, the movie ends on a note of melancholy self-reckoning that lingers. There’s a solid psychological drama in the well-acted and -directed Islands, waiting to be chiseled out in a tighter edit.