Like many John Le Carré novels, The Constant Gardener boasts a gripping, intricately plotted narrative that makes it ideal for the movies. In the years since the Berlin Wall tumbled and the Soviet Union collapsed, the master of the cold war espionage thriller has turned his attention to thorny moral issues in other parts of the world. The Constant Gardener is a muckraking drama set in an African country where the British have a monopoly over a corrupt pharmaceutical industry that, in its fervor to make a killing on a new antitubercular medicine, is using unaware native workers as guinea pigs.

That’s the shocking secret learned by Tessa (Rachel Weisz), the wife of low-ranking British Embassy official Justin Quayle (Ralph Fiennes). This knowledge gets her and a native doctor (Hubert Koundé) murdered in the opening minutes of the picture. The movie is about how Justin is drawn into the intrigue and, as a testament to his wife, makes it his mission to expose a conspiracy that not only implicates the knighted CEO of the drug company (Gerard McSorley) but also the British government.

Le Carré has always focused insightfully on the moment when personal morality breaks free of subservience to political loyalties. The film is about how a social conscience is born of an almost intolerable grief, and about how grief, rather than befogging Justin, brings him to a stunning clarity.

In an early flashback, we see Justin resist Tessa’s impulse to help some locals, explaining sympathetically that there are far too many disadvantaged people for them to attempt to rescue them all. Her response—that at least they can assist the needy right in front of their eyes—pits her idealism against his pragmatism, but in a clever and persuasive way. Tessa knows she can get somewhere with Justin, whom she loves for his kindness and patience.

The irony and tragedy of the story is that it’s her death that finally converts him. Near the end of the film, we hear him echo her words to his superiors, who have lost sight of the idea that they could wield their power for the good of the impoverished people rather than for those who want to profit from the misery.

This is the sort of material you would expect to catch the attention of Fernando Mirelles, the Argentinian director of the acclaimed City of God, a study of urban poverty. His ostentatious, jangling visual style—lots of flashy editing and hand-held camera shots repeated for effect—is the movie’s major liability, though it doesn’t obscure the complicated storyline or get in the way of the performances.

The extraordinary cast includes Bill Nighy as a high-ranking official (in his best scene, we get to see his impeccably rehearsed diplomatic delivery fall to pieces); Danny Huston as Justin’s boss, who oozes false sympathy; Richard McCabe as Tessa’s lawyer cousin, who adored her; and Pete Postlethwaite as an Afrikaner doctor whose inability to resist corporate temptation has shriveled his soul.

The heart of the film is Weisz’s portrayal of Tessa, though she appears mostly in the first hour and only in flashbacks. It’s one of those rare performances that continues to haunt the movie when the actor is off screen. Fiercely activist, yet tender and voluptuous in her scenes with her husband, Tessa is such a vivacious presence that she makes sense of what happens to Justin. For the movie to work you have to feel his compulsion to get at the truth about what happened to Tessa. Almost everyone else sees her as a naïve busybody, so we—along with Justin—have to fall in love with her. Weisz guarantees that we do.

Fiennes has a cold-fish quality in most movies, but here, playing a decent man whose professional weakness turns out to be the flip side of an unsuspected moral strength, Fiennes lets himself be vulnerable, and he’s very touching. Weisz keeps Tessa alive for us, but it’s Fiennes’s Justin who keeps her crusading spirit alive in the face of ugly and increasing opposition. At the end of the picture, you can feel him reach across the grave to grasp her outstretched hand.