As the title to Neel Mukherjee's first novel suggests, this is a story about never quite being a part of the worlds one inhabits. Having buried both his parents in Calcutta, Ritwik Ghosh arrives in Oxford, a scholarship boy studying English literature, and confronts dislocation and detachment. If this at first seems like familiar territory in the postcolonial novel of displacement, be assured that in Mukherjee's hands, it is a very much more original idea.

A number of things make this impressive debut stand out. Most strikingly, Ritwik is gay and Mukherjee (right) writes wonderfully and wryly about the young man's exploration of everyday gay life. Compulsively cruising the toilets of Oxford, Ritvik "realizes, in slow stages, that his is a type of minority appeal, catering to the 'special interest' group ... because of his nationality, looks, skin colour. He keeps pushing the word "race" away. The mainstream is blond, white, young, slim. Or... that is the desired mainstream."

There are some debuts which serve to announce a writer's promise, glimmering brightly between the flaws and missteps; and there are some follow-ups which more than confirm that promise as they make their way through increasing levels of complexity. It is rare, though, to find a debut which displays that promise amid flaws in the first half, and then soars into finely nuanced achievement in the second.

The first part follows a young Oxford student, Ritwik, recently arrived in England from Calcutta. Neel Mukherjee's deftness with moments that could easily be clichéd announces itself early on, as Ritwik fulfils that primary requirement of all fictional ­Indian characters who arrive in England – contemplating the rain: "For most of the time it is not the actual physical thing, the element of water, which he experiences, but the intent to rain, a sort of pervasive threat in the dead gunmetal skies."

The shift in the novel from uneven to assured takes place at the start of Part II when Ritwik, faced with the expiry of his UK visa, moves to London and becomes caretaker to the elderly Anne Cameron in exchange for a room. The growing affection between Anne and Ritwik is moving, surprising and entirely believable. She is the only real relationship he has as his growing penury and illegal status push him into a terrifying world of illegal work. The growing shadows and his new friendship are both given voice in Ritwik's continued reworking of Tagore's story, which takes in the partition of Bengal and simmering tensions between British and Indians as well as Hindus and Muslims.

A Life Apart is an elegant and accomplished debut, a novel of many shades. It blends the poignancy of a coming-of-age story with the rawer excitements of an urban thriller laced with sex and violence. It is only a shame that Mukherjee has gilded the lily, interspersing the bittersweet story of young Ritwik with extracts from a novel that Ritwik is writing — and which are far less engaging than the real thing. As so often, literary fiction proves its own worst enemy.