Justin Quayle, the amateur plant-fancier who takes the title in The Constant Gardener, is another hooded Fiennes fellow, a man more at ease with greenhouse cuttings than with the cocktail chatter that goes along with the post of a midlevel career diplomat stationed in Kenya. But such is the clarity and passionate intelligence of Fernando Meirelles' adaptation of John le Carré's urgent 2001 novel, about deadly pharmaceutical arrogance in Africa, that Fiennes blooms in his most empathetic, extroverted, and lovable work in years.

As in any le Carré creation, the players in this one are morally inconstant people who keep secrets and betray those closest to them. When, at the beginning of the saga, Justin's wife, Tessa (Rachel Weisz), is reported gruesomely murdered on an isolated stretch of country (she had been accompanied by a local doctor working to contain tuberculosis and AIDS, and the black-and-white pair had been rumored to be lovers), Justin himself really doesn't know what Tessa did with her days as a human rights ''activist.'' All he knows is that he adored her for her fire and for her commitment to fight, while he, a le Carré chap, after all, equivocated. (Weisz makes it easy to believe Tessa's fearlessness; she's as mobile, open-faced, and sexually alive as Fiennes is shuttered, and the two make a potent couple, even if the casting closes the generation gap that figures in the book.)

Tessa's death opens The Constant Gardener. Why she died (with damning information about multinational drug malfeasance in her possession) propels the plot, as Justin awakens from his complacency to understand the posthumous truth about her real work, and, poignantly, about their own marriage. Meirelles, meanwhile, brightens the author's wide, dark field with rich color, adding his own distinctive cinematic feel for the desperation, as well as the vibrancy, of the global underclass. Maturing the grabby style of hip-hoppy energy and visual fillips he brought to City of God (where he featured Rio de Janeiro as something between a circle of hell and a really cool setting for a music video), the Oscar-nominated Brazilian filmmaker shoots the landscape of Kenya, slums and magnificent wild territory, children and birds, with a new flow.

There's less superfluous ''art'' now, more concentration of purpose from frequent collaborator César Charlone's voluptuous cinematography. Now the shots don't waste time saying, Look at me; instead they urge, Look, look at what is really happening to millions of Africans as disease ruthlessly spreads, fertilized by a toxic mixture of corporate and governmental dither and greed. Working from an unobtrusive screenplay by British TV scripter Jeffrey Caine, Meirelles makes his points convincingly, teasing out the secrets of weak and bullying men (among the limpest of whom are Justin's unreliable friend and diplomatic colleague, played by Danny Huston, and their frighteningly Perfect British Boss, played with high gloss by Bill Nighy), but never squanders attention away from the tragedy of epidemic disease and corruption. (Playing the opposite of limp, the ever-excellent Gerard McSorley from Bloody Sunday embodies a paragon of corporate thuggishness who's never more terrifying than when he's on the golf course.)

There is, I realize, always the chance that such a serious, it's-good-for-you description makes The Constant Gardener sound like a lemonade glass of medicine. It's not. The movie is smart, serious, and adult about something that matters, but not at the expense of a kind of awful, sensual revelry as le Carré's capacious plot hurtles to its big finish. In flashback scenes of Justin and Tessa's life together, the chemistry between Fiennes and Weisz (previously paired in the tortured István Szabó film Sunshine) feels playfully sexy, which is not something I'd usually ascribe to Fiennes' default stance of proud hurt. Borne on lilting snatches of African song, The Constant Gardener gives a damn while giving good ''entertainment.'' P.S.: ''By comparison with the reality,'' le Carré explains in notes on the film, ''my story was as tame as a holiday postcard.''

John le Carre's densely plotted novels, which revolve around espionage, moral corruption and forces of evil at work around the globe, often flounder when transferred to the screen. Plots gets severely truncated and nuances are lost. Filmmakers try to cherry-pick the "cinematic" bits from the stories -- the cloak-and-dagger maneuvers -- but those separate with difficulty from the texture of his characters' lives and the thorough documentation of how rogues, governments and multinational corporations behave.

"The Constant Gardener" is a happy exception. One reason might be the inspired choice of Fernando Meirelles, the Oscar-nominated Brazilian director of "City of God," to bring the story to the screen. His impressionistic, guerilla style of filmmaking works surprisingly well in capturing the hypnotic urgency of le Carre's fiction. And his viewpoint is less British and more Third World. There are awkward moments, given the need to rush through a convoluted plot, and the peripheral characters that never fully come alive. But "The Constant Gardener" gets the essence of the story.

With Ralph Fiennes and Rachel Weisz toplining a work of clear passion, the film looks set for late summer counterprogramming as well as a competitive run with upcoming prestige offerings for Oscar nominations. Boxoffice should be steady though well short of blockbuster status.

The film, like the novel, opens with the death of a major character. Tessa Quayle (Weisz), a tireless political activist, is discovered brutally murdered in a remote area in northern Kenya. Her older husband is Justin (Fiennes), an ineffectual career diplomat attached to the British High Command in Nairobi, mostly concerned with tending his flower garden and keeping up appearances.

Initially, he takes the news with the apparent sangfroid of a true Etonian. Indeed it is Justin's associate, Sandy Woodrow (Danny Huston), who throws up at the sight of Tessa's mutilated body in the morgue, not Justin. Complicating his reaction is an indication that her murder might be a crime of passion: The Kenyan doctor (Hubert Kounde) with whom she was traveling has disappeared and is the chief suspect.

Justin then makes discoveries that could substantiate rumors of other infidelities by his young wife. But what no one in the community of expats in Nairobi counts on is the fierce love this man still has for the woman he scarcely got to know in their brief marriage.

The story moves in a nonlinear way as Justin turns into a mild-mannered bulldog, seeking an explanation for his wife's death. Then, in flashbacks, he examines more closely who his wife was. In the course of his confrontation with things he previously chose not to see, he draws closer to his wife; he understands her point of view, what mattered to her, and comes to love her even more.

This odyssey pulls him into the shady world of multinational pharmaceuticals or "pharmas" as the drug giants are called. These are organizations with enormous resources and economic power, virtual nations onto themselves, who think nothing about testing new drugs in the impoverished Third World.

Justin's investigation into what might have caused someone to order his wife's murder takes him into a scary and sinister terrain, where one feels no safer in the blazing light of day then in the mysterious dark of the night.

He visits Kibera, the largest slum in sub-Saharan Africa. In London, the British government confiscates his passport. He travels to Berlin with a fake passport to interview the scared head of a pharma watchdog group. He returns to Kenya to confront those with blood on their hands, then journeys to Sudan, where refugees live in vile conditions. The journey ends at the strangely beautiful site of his wife's murder.

(For all the criticism of the Kenyan government by the book and the film, the same government allowed the film to shoot in that country.)

The major disappointment comes in Justin's encounters with the crooks, thugs, spies, corrupt businessmen and Her Majesty's mendacious civil servants. These are played by such wonderful actors as Bill Nighy, Pete Postlethwaite, Nick Reding and Gerard McSorley. Yet they are all too familiar types. No doubt perfectly accurate types but le Carre -- adapted here by Jeffrey Caine -- is capable of creating characters with greater subtlety and dimension.

What distracts us from such things is Meirelles' arresting style that creates a vivid sense of place. Working again with cinematographer Cesar Charlone, the director overexposes some scenes, producing a kind of white on white. Meanwhile, in the slums and villages, as with the favela in "City of God," are a riot of deeply saturated colors. The camera jumps and tries to focus, as if a documentary film crew were shooting the film. Editor Claire Simpson keeps the story rushing forward as Alberto Iglesias' soft music, containing hints of African rock, pulsates in the background.