The Whistleblower is a film whose first act is such a mishmash of oft-done scenes and clunky exposition that it's a little surprising how engaged we are by its end. It stars Rachel Weisz, which is reason enough to see it, and it is (we are told in the opening title) "inspired by actual events," which is reason enough to hesitate. But it is a film that rewards patience; director Larysa Kondracki gains confidence and force steadily throughout the film's 112 minutes, and comes up with a pleasing and potent investigative thriller.

The primary problem with those early stretches is that Kondracki and co-writer Ellis Kirwan try to smash in so much set-up information that the scenes end up short and sorely short on nuance. We meet Kathryn Bolkovac (Weisz), a Nebraska cop and divorced mother whose daughter is moving (with her dad) to Georgia. Kathy is trying her best, but can't make the move with her; desperate for money, she ends up taking a lucrative job working for a private contractor supplying UN peacekeeping personnel in Bosnia (the film is set in 1999). She moves quickly through the ranks, but stumbles into something very big and very bad: a sex trafficking ring that is not only ignored by her fellow contractors, but might even involve them.

The process of meeting her, grasping her motivations, and getting her into place amount to a lot of table-setting; the side plot of her romance (with a married man, natch) is mostly a distraction, one that feels like it was left in primarily so that no one could accuse them of whitewashing the character by leaving it out. Throughout the first half or so, Kondracki works in a jittery, fast style that aims (and frequently succeeds) to help us overlook the somewhat slapdash storytelling.

But the picture starts to really, honestly work in a stomach-churning scene of Kathy exploring the bar (or, more accurately, the hidden dungeon inside it) where a prostitution ring has just been busted up--but probably not. The atmospheric use of light and shadow, and the horror of what she discovers there (and subsequently) propels the picture, which proceeds to gather force and energy as it goes.

And even in the less certain first half, it is impossible to overstate the value of Weisz's performance--her face, specifically, which carries the weight and shock of all that she sees and uncovers. You can see her thinking at every moment, as well as filling in the subtext and subtlety that is sometimes lacking. Her character--and her performance--gets angrier as the story progresses, and so does the movie. All are better for it.

Supporting performances are sharp--Vanessa Redgrave hustles into the movie and just gets right to it (in that wonderful way that few actors do), and David Straithairn, though somewhat underused, gives another of those wonderful performances where he tells you everything and nothing, simultaneously. The score is about as subtle as a crowbar to the head, but it works; the resolution, while relying on a mighty wheezy device, is altogether satisfying. Smooth and competent if a little shallow, The Whistleblower tells a powerful story, and provides a welcome showcase for one of our most intriguing actors.

Jason lives in New York. He holds an MA in Cultural Reporting and Criticism from NYU.