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I’ve got a soft spot for films with a simple premise and an existential purpose, where a single character’s battle to survive doubles as combat against an uncaring universe, their own sins, and even mortality itself.

Like most people, I’m also simultaneously thrilled and scared shitless by sharks; they are swift and implacable in an environment where we are sluggish and clumsy, all teeth, black eyes, and devouring death.

So I was all about the premise for The Shallows: Girl goes surfing in secluded bay. Girl gets bitten by Great White shark. Girl is stranded on a rock 200 yards from shore, with the monster still circling. Girl must figure out how to survive.

And that’s the whole shebang.

It’s starts off well enough. Nancy (Blake Lively) is being driven through the Mexican jungle to a bay whose name she doesn’t know. All she does know is her mother visited it when Nancy was conceived. Another friend was supposed to come along as well, but she’s nursing a brutal hangover. Nancy communicates all this to Carlos (Óscar Jaenada) her guide and driver, and their conversation is an intelligent, vibrant exchange of broken Spanish and broken English.

Director Jaume Collet-Serra uses some fun visual tricks to let us see things like Nancy flipping through the photo albums on her iPhone, revealing that her mother passed away from cancer. Later, similar effects show a FaceTime conversation between Nancy, her little sister (Sedona Legge), and her father (Brett Cullen). The latter is worried because Nancy has dropped out of medical school, and the trip to the bay appears to be part of some larger project of aimless wandering she’s embarked upon since her mother’s death.

The bay itself is an effective narrative instrument — a wonder of lush greenery, blue waves, multi-colored underwater geography. But it’s also a thin veneer of beauty marking the outer edge of a vast, dark, primeval underworld. Collet-Serra handles this dichotomy adroitly: as Nancy and two locals ride the waves, the film suddenly cuts between celebratory music-video-esque scenes of the surfers and silent glimpses into the murky depths.

Then Nancy goes out alone one last time as evening falls, and things turn ominous as all the animals go silent and she spots a dead whale carcass floating at the edge of the bay. Then the shark strikes, knocking her from her board and cutting a deep slice in her thigh. From there Nancy scrambles to the rocks.

This is where The Shallows loses its way. With a premise this bare-bones and contained, a filmmaker must rely on creativity and poetry to see them through. The Shallows does have a few interesting little twists. The roughest scene in the film is the unorthodox way Nancy closes up her wound. A GoPro from one of the shark’s other victims shows up, then unfortunately gets used in a manner that blatantly rips of 127 Hours.

At least twice, some element was introduced into Nancy’s world and I thought, “Oh, I bet I know how they’re going to use that later, and it’s gonna be awesome.” And then they didn’t. So the film leaves a few of Chekhov’s guns lying about, unfired. That wouldn’t be so bad if what the film did do was even better, but it’s not. The Shallows only proceeds in brute logic. Nancy’s final gambit is every bit as wildly implausible as the climax of Jaws, but I confess I didn’t quite understand the physical mechanics of it. And The Shallows lacks that film’s all-encompassing orchestral control of its audience to sell the moment.

Nor does Nancy herself have much of a character arc. She understandably remains passive during the first half of the film, hoping for intervention and rescue. Then becomes more active as time drags on, as thirst/hunger take their toll and her strength begins to ebb. She studies the shark, tracks its patterns, and plots tactics accordingly. But she never truly rises beyond the reactive — most all of her actions are driven by unexpected mishaps, or by the inevitable rising tide. Nancy passes through no great moment of despair, hope, revelation, or wonder in the midst of her fragility. The Shallows does stumble across the pieces for doing all those things, but never bothers to put them together properly.

The shark itself is reasonably effective. Collet-Serra and his screenwriter, Anthony Jaswinski, rightly never try to explain why it’s so hellbent on eating this particular woman. The shark is best when implied rather than shown: the fin slicing through the water atop the creature’s submerged bulk, or its monstrous shape circling in an overhead shot. It’s less intimating when seen in its full glory, and Collet-Serra relies too heavily on computer graphics to deliver needlessly over-the-top moments.

The music, which can be so important in a film like this is also a disappointment. Marco Beltrami proved his chops in other films, but here his score delivers a minimalist throb communicating only the most primitive emotional cues.

The strength of The Shallows’ premise is ultimately its undoing. Such a spare setup demands a high bar in terms of execution, yet Collet-Serra and Jaswinski just coast on rote thriller logic. The Shallows often looks great, and it will make you sink back in your seat and occasionally squirm. But given the task it sets for itself, that’s not enough.