Were it not for the nature of the imagery used in its pungent scene-setting, the opening of Little Rot, Akwaeke Emezi’s eighth book in six years, might easily be mistaken for romantic fiction. Young, affluent couple Aima and Kalu met in Houston, then returned to Nigeria together, where forgotten girlhood religiosity stirred in Aima. But Kalu, determined to resist social convention, refuses to get married, which is how he comes to be dropping her at the airport so that she can begin a new life in London without him. Instead of boarding her flight, however, she seeks out her “bad gehl” friend Ijendu, who prescribes that age-old remedy for a broken heart: intoxication and a strobe-lit dancefloor. With Kalu soon embarking on his own aching odyssey through the pulsing darkness of the fictional city of New Lagos, could an 11th-hour reconciliation be on the cards?

Perhaps, but descriptions of the setting sun as “an oily splash of colour, streaked blood in the sky under swollen clouds”, or the airport train that resembles “a rusting tapeworm”, filling the humid air with the screech of metal against metal, prefigure dark turns. Sure enough, the coming pages will deliver sexual violence, paedophilia and murder, as Aima and Kalu’s story expands over the course of 36 hours to include elements of an underworld caper and nihilistically inclined existential literary fiction. Holding it all together is a plot that ducks and dives with cinematic verve, gaining momentum and menace from a series of coincidence-fuelled twists that bind the fates of Emezi’s lovers and their privileged friends with sex workers, a closeted bisexual actor and the country’s most revered pastor.

The “rot” of the title emanates from New Lagos itself, a place of rank hypocrisy and corruption in which anything – and anyone – can be bought. Emezi has a flair for descriptive exuberance that is by turns poetic, melodramatic and scorchingly graphic, perfectly capturing the texture of this tainted world in which moral decay seeps into every fibre of life. Even a kiss involves a tongue that is “wet muscle”.

The action is fast and filthy, fuelled by shots and pills that don’t quite manage to dull the anguish of lost tenderness. Increasingly, notions of complicity intrude. “This city,” Kalu is cautioned by a stranger early on in that first evening. “You think you’re protected somehow, like the rot won’t ever get to you. Then you wake up one day and you’re chest deep in it.” Little Rot isn’t a perfect novel – Aima, for instance, blurs as a character, and cynicism snubs some of its deeper questions – but it bears out those words of warning with unflinching dedication.

“To those of us who cannot help but look at true things,” reads Emezi’s dedication. By the book’s close, even those accustomed to having the city’s darkest realities shielded from them by the tinted windows of their chauffeur-driven cars will have been forced to look. The reader, too.