The adaptation trail of “Girls Like Girls” is an unusual one. In 2015, pop singer Hayley Kiyoko released her hyper-catchy pop song of the same name, bringing a plainly worded statement of lesbian desire — “girls like girls like boys do” — into the viral mainstream, with an accompanying video laying out a compacted five-minute story of two suburban teenage girls discovering that their friendship is something more. Co-directed by the singer, the clip was artfully shot and empathetically told, and got fans sufficiently invested that Kiyoko eventually published a YA novel expanding the adventures of its young lovers, Coley and Sonya. And now we come around to “Girls on Girls” the movie, a full decade after the song’s first release — several eons in pop terms, then — but just as fresh and disarming in its articulation of queer self-discovery.

Not that you need know any of this background — or indeed anything about Kiyoko — to enjoy the star’s summer-soaked feature directing debut, which tells its unavoidably familiar tale of first love, first heartbreak and lessons learned with such open-hearted emotional purity that it feels new again. Or rather, it reminds you of when such feelings were new, and overwhelmingly big, as much as those older and supposedly wiser than you tried to tell you otherwise. The film’s two superb young stars, Maya da Costa and Myra Molloy, deserve much of the credit for its gentle, relatable warmth, but so of course does Kiyoko, who emerges here as a filmmaker of considerable skill and sensitivity — clearly capable of handling other projects not rooted in her own songbook.

Kiyoko and co-writers Chloe Okuno (“Watcher”) and Stefanie Scott — the lead actress from the original music video, as it happens — have opted for an early-2000s setting that may accommodate some millennial nostalgia (when apart, characters chiefly communicate not by text but by desktop IM messaging) but more pointedly serves to illustrate the extent to which young queer visibility has shifted in recent decades. Though it will play to young viewers who can’t conceive of life without a smartphone, “Girls Like Girls” should resonate with older LGBTQ audiences who grew up feeling alone in that identity, without many vocal peers or allies, and certainly without such normalizing cultural touchstones as “Heartstopper.”

Unusually, however, it’s not chiefly a coming-out story. 17-year-old protagonist Coley (da Costa) may be shy and unsure of herself in many ways, but the fact that she’s into girls isn’t a point of insecurity: She’s just quietly waiting to fall in love with one, and accepting that it may take a while. She’s a loner, after all, having only recently moved to a new town following the death of her mother, with a father (Zach Braff) whom she doesn’t know all that well. Summer stretches out before her like a blank diary, as she cycles in circles around the suburbs, bathed in the all-day magic-hour glow cast by DP Sonja Tsypin’s gorgeously hot, honey-dipped lensing, though not really enjoying it. Social rescue arrives in the form of outgoing cool girl Sonya (Molloy), who takes a shine to Coley after a chance encounter in a coffee shop, and invites her to join her clique.

Coley doesn’t much care for Sonya’s superficial friends, and certainly not for her boorish, territorial semi-boyfriend Trenton (Levon Hawke), but the girls click — when they hang out alone, which is increasingly often, the line between intense BFF affection and romantic love gets fuzzy fast. Kiyoko beautifully captures the charged rush of nascent desire, zeroing in on small, innocent gestures that, in the moment, feel seismic: the loan of a favorite jacket, the messages haltingly drafted and then scrutinized for subtext, the boundary crossed when one knee tentatively touches another in the back of a car. Crossed too far, at one point: Sonya may be preternaturally poised and self-possessed, but she’s still less accepting of her own sexuality than her gawkier would-be girlfriend.

Consumed with these tensions, and the will-they-won’t-they limbo of the girls’ relationship, the second half of “Girls Like Girls” is more expected and less seductive than the first — the film having begun as a woozy, sunburnt mood piece ambiently perched, like many an idle June afternoon, between reckless possibility and melancholic stasis. But it’s still moving and rewarding, illuminated throughout by da Costa’s wonderful performance, which conveys Coley’s depth and seriousness of feeling while also permitting the character spells of silliness and petulance.

She alternates between seeming far younger and more mature than Sonya, whom Molloy plays with compelling, casually volatile hot-and-cold energy; the film is enriched by a keen, specific sense of how both girls watch each other, sometimes passively and sometimes with rapt, unguarded fascination. Alive to both the soul connection and the bodily itch of these intimate, unwieldy, personally uncharted feelings, Kiyoko’s uncommonly lovely teen movie matches the dizzy, obsessive ecstasy of the song that inspired it — which plays over the closing credits in a new, slowed-down, blissed-out recording. “We will be everything that we’d ever need,” Kiyoko sings airily: a giddy line about first-crush idealism, from someone who lived to tell the tale.