Kathryn Bolkovac (Rachel Weisz), a Nebraskan police officer, accepts a well-paying UN peacekeeping job in United Nations-regulated Bosnia. Working for a private contractor, the International Police Task Force is responsible for training local law enforcement bodies to restore order to the wartorn country. As she begins to get the lay of the land in her new environment, Kathryn starts to see signs of a shocking underground human sex trafficking industry, whose patrons are not only from within the corporation but from within the United Nations itself.

A snapshot of a wider conspiracy.

At the age of 41 Rachel Weisz has been a film star of sorts for 15 years now. As is the dictates of Hollywood she’s been the underused female lead in a handful of commercially successful movies, including The Mummy, The Mummy Returns and Constantine, and survived misfires such as Envy, Fred Claus and The Brothers Bloom. She’s often the best thing in a lacklustre movie, and can mark About a Boy, The Shape of Things and The Fountain as credits to her name.

Yet for all that, she’s had until now one compelling role, as murdered political activist Tess Quayle in Fernando Meirelles’ remarkable 2005 drama The Constant Gardener, revealing a murdered woman defined by fierceness and passion in the memories of her husband, played Ralph Fiennes. Do you praise Weisz for surviving in an inherently misogynistic industry for so long or wonder why she’s taken so few risks with her talent? Whatever the answer, The Whistleblower is an attempt to do more, putting Weisz at the centre of a sometimes harrowing drama.

Based on true events – the name of Weisz’s character is real, while those of transgressors have been changed – Canadian filmmaker Larysa Kondracki’s debut feature follows Nebraska police officer Kathy Bolkovac, a divorced mother who in a bid to secure long-term access to her daughter absents herself to post-war Bosnia in 1999 to serve a lucrative short-term contract as a member of the United Nations rebuilding program. What she finds is a ruined landscape where despite the literal absence of local men, young women are being imported as sex slaves from other parts of Eastern Europe. Their clientele can only be the people charged with healing the damaged new state.

Cutting away to the story of several Ukrainian teenagers, including Raya (Roxana Condurache), the movie paints a grim and unyielding portrait of young girls turned into property, casually abused and kept in squalour, with acts of torture implied to an unforgiving degree. Kathy’s first reaction is to do her job, and what she encounters is not so much a conspiracy as a complete absence of oversight. Every U.N. employee has legal immunity, the local law enforcement is on the take.

In an atmosphere of boisterous machismo – the peacekeepers are like sex tourists who have day jobs – Kathy is stymied at every turn, with her guilty colleagues ridiculing her verbally with much the same physical condescension they treat the sex slaves with, while her superiors look away and rely on bureaucratic prevarication. The story, scripted by Kondracki and Eilis Kirwan, gives Kathy a mentor, in senior bureaucrat Madeleine Rees (played with her usual magnetic empathy by Vanessa Redgrave), but it never explains why those who do want to help can’t, or considers if that makes them accomplices by ignorance.

The picture focuses on the abuse, and Kathy’s struggle to stop it, and then to cope emotionally with her failure to do so. It doesn’t ponder the nature of the environment – you get little sense of Bosnia – or have a sense of evil’s reach; at a certain point Kondracki’s failure to create a resonant take, to deliver a visual representation of what Kathy is feeling, means that the abuse of the girls becomes close to repetitive and that Weisz’s command of her emotional extremes lacks genuine significance.

The Whistleblower does several things right, including putting aside the idea of the righteous American coming to cure corrupt Europe’s ills (the Americans, particularly fictionalised private contractor Democra Corp, are the cause), and it admits to Kathy’s own sexual needs, satisfied by a relationship with Dutch policeman Jan (Nikolaj Lie Kaas). But as in previous stories where vindication is never clear, the movie can’t find a satisfactory end point, making do with prologues that show Kathy on British television and adding text that makes clear her hitherto sketchy relationship with Jan endured.

The challenge here for Weisz is not, until a lacerating key scene, to overplay her performance, but to hold back. Her restraint offers a cold, evaluative gaze that makes what transpires all the more telling, and in the absence of direction that can make something more out of these elements than just condemnation, that becomes a strength amidst the small but accumulative flaws of The Whistleblower.