Starring Rachel Weisz and David Strathairn. Rated 14A. Opens Friday, August 12, at the Fifth Avenue Cinemas

With its thriller title and big-name star, The Whistleblower will catch many viewers off guard. Make no mistake: it digs into one of the most depressing, repugnant issues of our times.

The ugly reality is that UN peacekeepers have been linked to sexual exploitation around the world—in this case, in the former Yugoslavia. And the urgency of this message should help you forgive The Whistleblower’s occasional dips into sermonizing and melodrama.

The movie tells the story through Kathy Bolkovac (Rachel Weisz), a real-life Nebraska cop who headed to Bosnia in 1999 as a police monitor to solve her financial problems. When she stumbles on some badly beaten Ukrainian girls, she starts to uncover a sex-slavery ring that not only serves foreign peacekeepers but finds complicity amid the higher-ups at the private security firm who hired her.

Shot in Romania, The Whistleblower does not shy away from the grittiness of its subject, including harrowing (though never gratuitous) moments of violence against the terrified girls and vivid depictions of the syringe- and piss-pot–strewn bar/dungeons they’re kept in. It truly defies belief—until you start to Google facts about Bolkovac and her book (The Whistleblower: Sex Trafficking, Military Contractors, and One Woman’s Fight for Justice).

The film’s biggest strength is Weisz, who hands in a deglammed performance, tapping the blunt stubbornness of her heroine. Still, references to Bolkovac’s life back home—her divorce (“It’s not my fault you’re married to this job of yours!”) and the fact that she has daughters of her own—are trite. Equally frustrating: we get rich, believable back stories on the Ukrainian victims, but most of the male characters are depicted as broadly evil. So who were the assholes that did unthinkable things to girls? The fact that Brits, North Americans, and others were implicated demands deeper exploration.

Canadian director and cowriter Larysa Kondracki’s film works better as a straight-up thriller, following the naive cop as she only makes things worse by trying to serve justice. Still, the nauseatingly grim subject matter and complicated politics don’t exactly lend themselves to a mainstream movie. The world has largely averted its eyes from the issue of human trafficking, and audiences, unfortunately, may choose to as well.

Watch the trailer for The Whistleblower.