Abdellatif Kechiche’s Mektoub, My Love is a 90s summer-romance pastoral of epic length and sexiness, marinaded in sunshine. It spends its time among unfeasibly beautiful young people in microscopically tiny swimming costumes, and moves with them in a trance of heightened physicality, drifting across beaches, bars and dancefloors. The mood is dreamy unseriousness qualified occasionally by stabs of jealousy or misery. The sexiness isn’t promiscuous exactly; more directionless. There is a sustained hedonism here: it floats along and you will wait a long time – three hours in fact – for something to happen in the boringly conventional narrative sense, some sudden event that will dramatically or ironically cut across all this sensuality.

It is adapted from the 2011 novel La Blessure, La Vraie, or The Real Wound, by François Bégaudeau – whose Entre Les Murs was the basis of Laurent Cantet’s Cannes Palme-winning movie The Class from 2008, starring Bégaudeau himself. The year is 1994 and Amin (Shaïn Boumedine) is a young guy who has abandoned his medical studies in Paris and come back to his hometown: the Mediterranean port of Sète, which was the setting of Kechiche’s earlier work Couscous. (There’s a sly early nod to that film.) Now Amin has got nothing to do but hang out, all summer long.

So why did Amin come home? Partly it’s because he needs time to think about the new career direction he’s fixed on – being a screenwriter, or maybe photographer. But Kechiche makes it clear Amin is still emotionally embedded in the ongoing soap opera of those love lives he left behind and the rich, garrulous warmth of his extended family who run a Tunisian restaurant locally.

Is Amin himself in love? Well, he appears to have strong, unexpressed feelings of some sort for Ophélie, a smart debut performance by black-belt mignonne Ophélie Bau. Ophélie is supposedly engaged to a guy called Clément who is in the military, doing service overseas, but Amin has come back to discover she is having an affair with his cousin Tony (Salim Kechiouche), a commitmentphobe, heartbreaker and all-round party animal.

Amin’s summer of love will consist of hanging out with Tony while he ostentatiously seduces other women, as part of a diversionary tactic to draw gossips away from his liaison with Ophélie – a scheme to which Ophélie has semi-assented, being still notionally committed to Clément. It creates a crosscurrent of anxiety and resentment that ripples under the party atmosphere. Tony puts the moves on tourist Charlotte (Alexia Chardard) while Amin is supposedly paired off with her friend Céline (Lou Luttiau). The excellent Hafsia Herzi, from Kechiche’s Couscous, returns as Amin’s aunt Camélia, and the director’s sister Delinda Kechiche and Hatika Karaoui give lovely, warm performances as Amin and Tony’s mums.

The sex in the movie is constructed counter-climactically. Instead of beginning with lots of tension and building to a lovemaking scene, Kechiche opens with full-on sex and lets the sense memory of that roll forward, all the way through every subsequent moment of simmering attraction between various people. Even more than in his previous picture, the Cannes Palme-winning Blue Is the Warmest Colour from 2013, Kechiche has revealed himself to be a real enthusiast for unclothed attractive young people. Eric Rohmer was famous for showing twentysomethings in swimming costumes, but I don’t remember Rohmer’s camera lingering quite so rapturously on young women’s bottoms. Complaints about the male gaze, which were targeted at Blue Is the Warmest Colour, may be revived here.

Two key scenes take us away from the loved-up ambience. Amin begins his summer by sitting around in his bedroom watching classic movies on video: including Alexander Dovzhenko’s 1929 silent Arsenal – the macabre scene in which a German soldier in 1917 is driven crazy with laughing gas. Weirdly, it often seems as if everyone in the later club scene has imbibed some sort of laughing gas: they’re just always laughing as they dance, flirt, drink and flirt.

Then there’s another sequence – held by Kechiche, like every other scene, for a great length of time – when Amin hangs out at Ophélie’s family farm and photographs lambs as they are being born. That is a great scene, in its way, though perhaps cannot work as an epiphany quite as fully as Kechiche evidently wants it to.

Mektoub, My Love is reportedly to get one or even two sequels: a franchise of twentysomething ardour that might extend all the way into the 21st century. For all its exuberance, this is a film that is sometimes on the verge of submitting to softcore erotic torpor. I want the next movie to wake up.

• This article was amended on 14 February to correct an editing error.