It's a story of It-girl vs Shark - Blake Lively, whose celebrity still seems to outweigh her screen credibility, as a surfer stranded on a rock and other things by a Great White down Mexico way.
But it is a B-movie hoot big on suspense and curiously low on gore.
Director Jaume Collet-Sera, who was behind the Liam Neeson actioners Non-Stop and Unknown, lays his visual style on thick and with seeming ADD, mixing random slow-motion shots with GoPro found footage, as well as some effective jump-scares and an excess of ogle-cam.
Nancy stars as Blake Lively in the new shark movie The Shallows.
A bikini-ed Lively spends a good deal of the movie wriggling in and out of her wetsuit top or paddling away from the lingering lens.
It's like Collet-Sera has taken the opening scene of Jaws - the one of the skinny dipping woman becoming sharkbait - and made a whole guilty pleasure film out of it.
No wait, there's a back story too. Nancy's mom came to this same remote beach when she was pregnant with her twentysomething years ago.
Now that mum has succumbed to cancer, Nancy has quit med school to make the pilgrimage and catch some waves. So there's perfectly good motivation to her tourist-foolhardiness. She's doing it for mum.
Aww. And hey, it's a nice spot and the locals are friendly. Bar one notable exception, which despite an entree of whale and a side of surfer dudes, is still intent on giving Nancy a gnashing.
After a run-in with the shark, she ends up on a rock 200m from shore with the tide rising, losing blood from a leg wound she's been able to stitch up due to that aforementioned medical training and some sharp jewellery.
Good thing she didn't leave those earrings in her backpack on the beach where she left her phone, on which she video-called her worried sister and father before paddling out (Hey lady, get yourself one of those waterproof ones for next time huh? Ask the guys in product placement).
Lively's character may have to do some dumb things and spend a fair amount of time looking like a wounded mermaid on that rock while chatting to a seagull, but you have to admire her physical commitment.
The scrambling from one above-water sanctuary to another kind of works like an oceanic version of Sandra Bullock's space-capsule hopping in Gravity.
A bikini-ed Lively spends a good deal of the The Shallows wriggling in and out of her wetsuit top or paddling away from the lingering lens.
And Lively does enough to make you believe she's a smart and resourceful woman who is doing her best to get out of the stupid spot she's got herself into.
That means making for an anchored navigational marker 20m way. Will girl meet buoy? Will buoy rescue girl? And those storm clouds gathering out to sea? Surely that's not a Sharknado?
Pondering stupid thoughts like those is half the fun of The Shallows. The other is getting caught up in the tension on the way to its entertainingly ludicrous finale.
The Shallows
Cast: Blake Lively Director: Jaume Collet-Sera Rating: TBC Running time: 87 mins Verdict: Enjoyable, if you're willing to take the bait
