Thanks to a killer Blake Lively performance and a poignant seagull, director Jaume Collet-Serra finally earns his cult.

As Jean-Luc Godard famously never said: “All you need for a movie is a girl and a great white shark.” What “The Shallows” presupposes is that adding a cute seagull, a rotting whale, and a few GoPro cameras to the mix probably wouldn’t hurt. Unequivocally the best shark movie since “Jaws” (yes, even better than “Open Water” and “Deep Blue Sea”), this back-to-basics thriller either eliminates or reclaims all of the excess and gimmickry that have watered down the genre since Steven Spielberg first invented it — there’s only one killer fish, she’s shot in beautiful 2D, and it doesn’t appear as though the beast has developed the ability to swim backwards as the result of reckless genetic modifications. The film flirts with found-footage, but only in small and supremely effective doses; the shark is a digital effect, but a glorious one whose artificiality is only clear in a few crucial shots towards the end. With “The Shallows,” Spanish director Jaume Collet-Serra (“Non-Stop,” “Orphan”) finally earns the cult that’s been champing at the bit to praise his recent work.

Nancy Adams (Blake Lively, in the most convincing argument for her stardom since 2009’s “The Private Lives of Pippa Lee”) is a med school dropout with nothing to lose beyond her surfboard and her cell phone. Introduced as she’s being driven to the secret Mexican beach that her late mother visited when Nancy was in utero, our glassy-eyed Galveston girl is on a solo mission to commune with her past and make sense of the loss that continues to haunt her. It’s an unformed and ill-advised plan, motivated only by the emotional logic of helplessness — what’s the point of becoming a doctor if you can’t save your mom from cancer? What’s the point of living if every road ends in death? That might sound like a rhetorical question, but nothing inspires an answer quite like a prehistoric killing machine that doesn’t give a soggy fuck about your grieving process.

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Budgeted at a relatively tight $17 million, “The Shallows” knows that a movie only needs one shark if it packs a strong enough bite. Far more focused on suspense than scares, the film balances the crowd-pleasing thrills of a true summer blockbuster with the patience and painterliness of less disposable entertainments. The build-up to the shark’s beautiful first appearance (which chums the water with two Mexican surfing bros who give Nancy a hard time) is masterful stuff, as Collet-Serra lingers on Flavio Labiano’s gorgeous ocean cinematography and subtly ingrains the film’s geography into our minds’ eyes.

No, it doesn’t take a cartographer to appreciate the relationship between the shore and the jagged rock — some 200 yards out to sea, and completely submerged during high tide — on which Nancy will have to huddle for dear life after the great white takes a bite out of her leg, but Collet-Serra ensures that we feel the risk of every stroke between his heroine and her safety. The action is visceral and immediate, but crucially contextualized by a helpful array of wide shots and bird’s-eye views.

It helps that so much of this savage duel between woman and shark is rooted in a believable physical reality. Lively throws every inch of her long frame into the role; the camera inevitably eyes her like a piece of meat (all the better to appreciate the shark’s POV), but the actress manages to carry the whole movie on one leg, fending off all sorts of hungry looks as she blossoms into a soulful female MacGyver.

Coral, jellyfish, and the rotting husk of a dead whale all play key roles; at one point, Collet-Serra turns a buoy into its own location, exploiting every possible use for the bobbing metal beacon in a way that recalls the joy of watching Jackie Chan make the most of his battle props. Best of all is how the movie elevates one of its decorative touches — a stranded seagull with a broken wing — into a bonafide supporting character (like Wilson the volleyball, but cuter). The bird is such a delightful screen presence that “The Shallows” could do for seagulls what “City Slickers” did for calves; at the very least, you’ll be a little more forgiving the next time one of those flying squawk monsters shits on you at the beach.

Collet-Serra isn’t in the business of making high art, but his trash is crafted with a degree of care and cohesion that’s lacking from many of Hollywood’s proudest awards fare. “The Shallows” is a modest film even for a late June diversion, but — with the exception of a limp coda and a few inelegantly framed text conversations — this thing is all killer and no filler. Nancy’s ordeal may not pack the same gravitas as other recent genre movies about death (“The Babadook” springs to mind), but the shark is less a metaphor for grief than it is a mega-ton argument against giving into it. We’re all helpless in the long run, but that doesn’t mean we can’t save ourselves.

Grade: B+

“The Shallows” opens in theaters on Friday.

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