FERNANDO MEIRELLES’s hair-raising exposé of an international drugs scandal is the most dramatic opening gala I’ve seen this year. It has all the ingredients that make The Times bfi London Film Festival such a special event.

Based on John le Carré’s thriller, the film is a blistering attack on pharmaceutical malpractice in Kenya, ingeniously coupled to a harrowing romance. It opens with the brutal rape and murder of a British diplomat’s wife, Tessa (Rachel Weisz), and spirals into a gripping quest by her shattered husband, Justin (Ralph Fiennes), to discover the reasons.

The gruesome evidence points to a crime of passion, but Tessa’s secret life as a campaigning activist pricks her husband’s suspicions. Fiennes has rarely been this magnetic on the big screen. His diplomat is stranded between genuine grief and a Foreign Office that wants to sweep the whole beastly affair under the nearest carpet.

A box of Tessa’s personal letters reveals hints of a corporate conspiracy involving a ruthless pharmaceutical company, randy Whitehall mandarins, unscrupulous investors and dodgy African politicians. The lucrative race to find a cure for tuberculosis — by testing potentially fatal doses of a new drug on unsuspecting Kenyans — rings horribly real and true. You can sense this outstanding film’s bottled anger at the West’s lopsided relationship with Africa.

Justin’s colleagues at the British High Commission are spookily evasive. The stakes are unspeakably high. And his suicidal interest in finding his wife’s killers is as hard to explain as his marriage.

The film sifts the memories of Justin and Tessa for clues. The diplomat is passionate about garden plants, not his job. His shapely wife flirts with activists at their stuffy Nairobi parties and dangerously insults the rotten dignitaries.

What makes the pair click is as enthralling as the political twists. Danny Huston is wonderful as Justin’s sweaty boss, who tries to bed Tessa. And Bill Nighy is fabulously insincere as the minister who tries to pin the murder on an infatuated doctor. “Albert is homosexual,” says Justin. “Well, I’ve known one or two savage queens in my time,” quips Nighy.

Meirelles assembles the film like a jigsaw puzzle. Flashbacks to intimate moments are clues to the scandal. Memories are slipped between tense conversations with frightened witnesses in the bush, slippery officials in London and unsubtle thugs in Berlin. It’s a love story bathed in grief and spiked with satire.

The Brazilian director is an inspired choice. His stunning camera work captures the beauty of Kenya and the harshness of the streets, without over-egging a frame. A serious joy.