Equator, by Miguel Sousa Tavares, translated by Peter Bush

Passion at zero degrees

Portugal in 1905 is a place of dramatic progress – social reform, a rampant anti-monarchist movement, the arrival of the motorcar. But this progress does not extend to the country's distant colonies; there are no such advances in São Tomé e Príncipe, a pair of islands stranded on the equator west of Africa, drifting nowhere in the Atlantic.

These islands survive on cocoa exports, with wealthy plantations manned by Angolan workers who toil in the equatorial heat and humidity. An important question arises: the conditions may be dreadful, but do they amount to slavery? The workers are salaried, on contracts, but are they truly free? Suddenly the answer matters to the politicians; and for the plantation owners the stakes are high, for if the newly deployed British Consul finds that slavery exists, "a British company called Cadbury" will cease importing their cocoa and São Tomé's economy will collapse.

Enter Luís Bernardo Valença, emissary from King Dom Carlos, despatched from Lisbon to smarten up labour practices and to persuade the British that all anti-slavery agreements are being observed. Valença is persuaded to leave his Lisbon life – the family shipping firm and gentlemen's clubs – to spend three years in tropical torpor, bereft of friends, opera, cigars, alone in a hostile environment.

Equator is about colonial power and responsibility, and the tensions between the agenda in the imperial capital and real life in the satellite colonies. The theme is old, but always relevant: the clash of value systems, and the impossibility of forcibly overlaying one society on another. Tavares dresses the atmosphere with detail from discreetly-worn research that populates the novel with real historical figures. But it is the fictional story that is at its heart. And through the prism of Valença's dilemmas, Tavares brings into focus the moral complexities with an admirably light touch.

What is lost through the focus on a single figure is interest in other characters; Valença's amorous conquests, particularly, are all impossibly appealing; and while the book does have one substantial section on the back-story of British Consul David Jameson, it comes at the cost of breaking the narrative.

Equator is easy to read, and to like; the settings are consistently engaging. And simple vignettes brilliantly convey the troubling questions at its heart. Valença awakes at dawn on a plantation to hear the workers singing, a sublime moment followed by his tour of the plantation's exemplary hospital facilities, where "everything was perfect, except for the odour of formaldehyde and death".

The prose is fluent, in Peter Bush's largely invisible translation, and the good story well told. Because for all the historical detail and moral debate, it is above all a gripping story, packed with blackmail, scandal, a passionate affair, gambling debts, a violent workers' revolt, intrigue, heartbreak, a courtroom drama and a gunshot in the night... Old-fashioned stuff, but hard to resist.

Daniel Hahn's translationof Jose Eduardo Agualusa's 'My Father's Wives' is published by Arcadia