Coming off the riotous, outraged exploitation picture that was City of God, Fernando Mereilles turns relatively sober. The Constant Gardener is a more ambitious, Hollywood-style thriller about Big Pharmacology up to no good in sub-Saharan Africa, where life is cheap because the Western world turns a blind eye to corporate shenanigans. It’s also a broken love story that begins at the end of a marriage, with diplomat Justin Quayle (Ralph Fiennes) saying goodbye to his wife Tessa (Rachel Weisz) for what he doesn’t know will be the last time. As Justin begins examining her life and death, he starts examining his own principles, and wondering how many people he’s betraying through his own inaction — essentially, he’s part of an ineffectual bureaucratic machine that has the stated purpose of ensuring that foreign aid is properly dispensed but fails to examine the real relationship between the poverty-stricken locals and their alleged corporate benefactors. He’s the gardener of the title, tending to the carefully arranged flora outside his tasteful home, just as minor bureaucrats all over the world tend in superficial ways to the affairs of developing economies and underprivileged peoples within their spheres of influence.

The political intrigue here is still pretty scandalous, given the sorry state of the health-care system in poverty-stricken Africa and the area’s continued ripeness for exploitation by big economic interests who can pull out of it much more than they put in. (The film’s most chilling moment is probably an end-credits epigram in which John LeCarré, who wrote the book on which the film is based, claims the real-world behavior of pharmaceutical companies that he researched makes his own fictional yarn look like “a holiday postcard”.) But as a political thriller, The Constant Gardener is a little clumsy, Maybe it’s the result of trying to cram a LeCarré-sized narrative into the temporal strictures of a feature film, where the revelations have to tick along like clockwork even though none of them really feel like revelations — once the story is set in motion with the murder of Justin’s wife, it’s not much of a jump to suspect she was murdered for asking the wrong questions and embarrassing the wrong people. Indeed, flashbacks quickly fill us in on her status as an activist dogging the powers that be. Tessa actually meets Justin when she shows up at his speaking engagement to harangue him with a rehearsed diatribe against British policy in the Iraq War. It’s completely well-intentioned but also embarrassingly gratuitous — it plays like something cooked up and dropped into the screenplay to establish her credentials in feistiness.

From there, it’s just a convenient hop, skip and jump until (of course!) the two of them end up in bed together, madly in love. Justin eventually takes her with him when he’s assigned to Nairobi, where she establishes herself as a veritable Mother Theresa, befriending the children, advocating for the infirm, and working quietly in concert with a local doctor to reveal the corporate chicanery that’s leaving a trail of dead Africans in its wake. (The movie’s black joke is that nobody in the West cares much about dead Africans.) How and why does the passionate lefty fall for the bland, doing-my-job government functionary? Well, that’s never explained. That’s convenient, because Mereilles wants us to wonder for a while whether her marriage to Justin is a sham, designed to move her into a position where she can more closely observe the machinations of the big drug companies.

As a thriller, this is mighty perfunctory. Each scene depicts an event or a conversation that serves to advance a story thread or simply provide a piece of exposition, and the dialogue often plays as stagy and pretentious. As love story, it’s contrived and mopey and, at its worst (I’m thinking of the radiant flashbacks of blissful days with his wife that are replayed as a love montage as Fiennes wails and grimaces in her backyard garden)  a little maudlin. As travelogue, it’s remarkable — it’s creatively photographed, and digital tweaking has made the images of the African desert almost hallucinatory in intensity. There are sequences in The Constant Gardener that skillfully combine those three impulses, and they crackle with life. But mostly the narrative feels disconnected from itself, lacking vigor and conviction. And boy, does this movie need Rachel Weisz, who’s terrific in it. As long as she’s on screen, the proceedings are lively and human — OK, except for that weird prosthetic pregnancy she carts around in her nude scenes — even though she’s portrayed as an impossibly savvy, virtuous and beautiful woman. (Justin’s investigation seems to specifically clear her of any imagined betrayal.) And once this unambiguously kind-hearted soul is gone for good, Fiennes bears such a burden of self-loathing that the film droops and sputters to conclusion. It’s interesting and absorbing enough that I can’t call it a failure, but it doesn’t cohere enough to qualify as a success. In short, Mereilles hasn’t embarrassed himself, but he can surely do better. I’m rooting for him to find another project that makes more organic use of his strengths — a strong sense of place, a loud aesthetic, and a ferocious voice.