Anyone who is afraid that the English novel is sliding into a backwater of domestic anecdote should find their anxieties assuaged by the writing of Michael Arditti. He has tackled big, disturbing themes in his earlier novels, including The Celibate, Pagan and Her Parents, and Easter, but in his latest fiction, Unity, he addresses perhaps the biggest and most disturbing question of all.

Unity is the more unsettling for being presented in documentary form. The narrator, “Michael Arditti”, is, like his alter ego, the author, a Cambridge graduate who attended the university in the mid-1970s before becoming a novelist. The novel takes the form of an inquiry by “Michael Arditti” into the strange and violent destinies of two of his fellow undergraduates, Luke Dent and Felicity Benthall — a fate in which “Michael” himself might easily have been implicated had it not been for a chance turn of fate.

The story begins with the undergraduate friendship between the three main characters, who meet at an audition and begin one of those ambiguous, multi-faceted attachments, poised between friendship and love, that were for many undergraduates of that period the most significant learning experience of their time at university — the life lessons thus learnt far more durable than any tutorial discussion. (“Michael” defines a university education as “the pursuit of knowledge in the company of friends”.) Felicity, the granddaughter of a baronet, and Luke, child of an embittered expatriate businessman, become lovers. “Michael”, who is gay, occupies a more ambigious role in the triangular relationship. He sleeps once with Felicity (“it was more of a biology lesson than a romantic tryst”) and not at all with Luke. “I am, however,” he writes, “the one who wrote the inscription on his grave.”

The three devise a Cambridge Footlights show based on the life of Unity Mitford, the Nazi-loving sister of Nancy, Diana, Jessica and Deborah Mitford, who trumped her sisters’ various notorieties by her public obsession with Hitler, and by attempting suicide when war was declared betwen Germany and Britain. Luke wrote the script, Felicity played Unity and “Michael”, Hitler. Among the audience one night there appears the notorious German film director Wolfgang Meirs, an “identikit iconoclast”, who whisks Luke and Felicity off to dinner after the show, excluding “Michael” — to his disgust — both from the dinner-party and from his subsequent plans for a film of the drama.

While “Michael” retreats, post-Cambridge, into thankless schoolmastering, Luke keeps in touch from the film set in Germany, where Unity is being shot, in a series of letters describing the strangeness of Meier’s film set, and the unsettling network of relationships among the cast and crew. among which a powerful sense of Pandora’s box is beginning to prevail, fuelled not just by the Nazi history that forms the substance of their film, but also by the far-Left political radicalism of 1970s Germany.

Eventually, the drama on screen merges with an equally destructive real-life drama, which “Michael Arditti” determines, 30 years after the event, to do his best to anatomise by bringing together Luke’s letters with other records of the time — the diaries of the actress playing Diana Mitford, and his own interviews with surviving members of the cast. In the course of his researches it becomes plain that what he is trying to understand is not merely the story of what happened to his friends, but also the nature of human destructiveness.

Strikingly original in form, its multiplicity of different voices rendered with a ventriloquist’s eerie skill, Unity is a remarkable, unsettling book, both a compelling fiction and a troubling inquiry into the nature of fanaticism.

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