When Kathryn Bolkovac, Nebraska police officer and divorced mother of a teenage daughter, signs on to a high-paying job with the United Nations peacekeeping force in Bosnia in 1999, she looks for a fresh start.

What she finds in the squalor of Sarajevo is deeper-rooted and more degrading than ethnic hatred. Even worse, the perpetrators may very well be her colleagues.

The Whistleblower, Larysa Kondracki's uneven debut feature exposing the explosive subject of sex trafficking, derives its strength from Rachel Weisz's intelligent performance as the real-life Bolkovac.

The empathic professional discovers cells of indentured, mostly underage sex slaves smuggled in from Ukraine. Many are younger than her daughter. Eyes glassy from abuse, they spend sleepless nights chained to pallets in the basements of bars and clubs. "War whores," her colleagues call them.

But the war is over. "Half of our men are dead," says a Bosnian woman sheltering runaway slaves. "So, who do you think these girls are for?"

Bolkovac resists the implication that it's the men working as peacekeepers and rebuilders in the former Yugoslavia who are clients of the traffickers pimping these girls. But each thread of evidence she picks up leads to an even more tangled, and disturbing, possibility.

The story, inspired by Bolkovac's experiences in Bosnia and her subsequent book account, is dynamite. Alas, Kondracki's direction fizzles. While she elicits a tense and eloquent performance from Weisz, the first-time filmmaker fails to maintain a consistent tone. Her film samples multiple genres.

The handheld scenes of sex workers look like they were made by an exploitation director. The sequences of Bolkovac investigating the sex traffickers look like outtakes from a horror movie. When Bolkovac stands up to her sexist bosses, Kondracki's film sweats like a thriller. Because of the abrupt tonal shifts, Kondracki does not maintain momentum. Despite this, Weisz does.

The Whistleblower **1/2 (out of four stars)

Directed by Larysa Kondracki. With Rachel Weisz, Vanessa Redgrave, Monica Bellucci, and David Strathairn. Distributed by Samuel Goldwyn Films.

Running time: 1 hour, 52 mins.

Parent's guide: R (graphic nudity, pervasive sexual violence, profanity).

Playing at: The Ritz Five.EndText

Contact movie critic Carrie Rickey at 215-854-5402 or crickey@phillynews.com. Read her blog, "Flickgrrl," at http://www.philly.com/philly/blogs/flickgrrl/.