At a time when more and more promising directors are quickly swallowed up by the remorseless blockbuster machine, there’s something admirable about a filmmaker like Jaume Collet-Serra (Non-Stop, Run All Night), who is seemingly content to stay a rung or two down on the respectability ladder and continue refining his chops. The Shallows, Collet-Serra’s new primal screamer, may not be his best work—that honor still falls to the wonderfully sick Orphan—but its single-minded devotion to getting viewers to grip their armrests is really something to see. Clocking in at a lean and mean 86 minutes, it takes its deliberately simple premise and comes close to knocking the damn cover off of it.

Anthony Jaswinski’s script has a high-concept purity: Erstwhile med student (Blake Lively) heads to a remote Mexican beach to catch some solo waves, only to discover that the water isn’t quite as empty as she thought. (Okay, you’ve seen the trailer, it’s a huge freaking shark.) Lively is almost the whole show here, and thankfully she proves to be more than up to the task: intensely physical, charming in her brief interactions with some notably unfortunate passersby, and believably self-reliant throughout, even as her situation steadily worsens.

Movies like this demand a showdown, and the final prolonged confrontation between Woman and Nature does admittedly get a bit silly, especially when compared to the creeping, ingenious dread that came before. Even at its most overt, though, The Shallows shows just how bracing and propulsive a good B-movie can be, especially in the hands of a director who knows exactly when to linger on a shot of the indecently beautiful scenery, and when to dip the camera below the waterline for maximum unease. It’s pulp, but artful pulp. recommended