THE fall in question was a fatal one for the narrator's older brother.

The shock of it haunted the younger boy for years.

The accident happened late one night when the two boys – Matthew and Simon – surreptitiously ventured outside their family's caravan at the Ocean Cove Holiday Park in Dorset. Matthew, just nine years old, has coaxed Simon out of bed and taken him to a spot where he saw something interesting happening earlier. The episode ends unexpectedly, tragically.

Nathan Filer's vivid debut novel opens with Matthew, now 19, looking back at the family's time at the holiday camp, though not at the fatal accident itself, which is, instead, alluded to throughout. It is plain that Matt blames himself for the accident, and that this has had a serious impact on him. In his own words, while writing about himself in the third person, he "suffers from command hallucinations, which he attributes to a dead sibling".

He is "managed" by his local community health team, attends therapeutic groups, receives medication. He refers to himself as "schizophrenic" at one point. You wonder whether he will ever be able to shake off the dread that he was responsible for his brother's death.

Simon, it emerges, had had special needs. He had a weakness of the muscles. Matthew consults a nursing dictionary in the day centre he goes to ("Is it appropriate for patients to borrow the dictionary?" a student social worker asks a more senior colleague), and looks up the necessary words. "Simon had hypotonia," he writes. "He also had microgenia, macroglossia, epicanthic folds, an atrial septal defect, and a beautiful smiling face that looked like the moon."

Matthew brings his story forward, slowly. His mother home-tutored him for a while; he refers to her taking him out of school and isolating him at home for reasons that got lost in her grief. By fits and starts, Matthew tries to make a life for himself. He shares a flat with a school friend, Jacob; but Jacob has problems of his own at home.

Eventually, as we know we will, Matthew takes us back to the accident itself – an incident that unfolded within the space of a few seconds and would go on to have such a debilitating impact on his own life. We can almost picture Simon in his last moments, his 12-year-old life in its dying fall. "Whatever wave had been swelling in the sea in the seconds before he fell," Matthew writes, bleakly, "would break in the seconds after. This dismissive and uncaring universe simply carried on with its business, as if nothing of any consequence had happened."

Filer, who in addition to being a registered mental health nurse is a performance poet, does a skilful job in getting under the skin of Matthew and staying there. Matthew at times seems like any other stroppy teenager, which might try your patience, but he is no normal teenager; and as he concedes late on in the book: "Mental illness turns people inwards - I'm stuck looking inwards. Nearly every thought I have is about me – this whole story has been all about me; the way I felt, what I thought, how I grieved."

A interesting device in this 300-page-long novel has Matthew writing out his story on an office computer or on an old, charity-shop typewriter, given him by his grandmother, the switch between the keyboards denoted by a change in typeface. There are occasional line drawings, too, which gives the book the feel of a personal journal.

Filer has a perceptive ear for dialogue, particularly the small talk we all lapse into and the conversations between Matthew and his father. The Shock Of The Fall is a convincing portrayal of an articulate boy in the grip of a powerful illness, and an absorbing account of a grief undischarged and a sense of guilt that has been allowed to fester.