Eleven-year-old Byron Hemmings becomes anxious when his friend James tells him that two seconds are going to be added to the clock to compensate for the 1972 leap year. Fretting about this when his mother Diana is driving him to upscale Winston House school one morning, Byron is sure his watch has moved backward. He insists on showing Diana the watch, which causes her to swerve her Jaguar and hit a young girl on Digby Street - a neighborhood of working class people. Unaware of what's happened Diana continues on her way.
Byron, obsessed with the accident, hounds Diana until he convinces her of the incident and she goes back to Digby Street to "confess". This starts a series of events that have dire consequences. Diana develops a friendship with Beverley, mother of the injured child Jeanie. Jeanie sustained very minor injuries but - as Beverley becomes more and more envious of Diana's lovely home and lifestyle - Jeanie's "disability" suspiciously become worse and worse. This, in turn, makes Diana more and more frantic to make amends.
Byron, wanting to help Diana and encouraged by his friend James, studies what's going on and keeps a journal where he writes and sketches everything - starting with the accident and continuing with Beverley's visits to his home, Jeanie's escalating problems, and so on. He shares this observations with James, who seems to be over-interested in the entire affair.
All this exacerbates the tension in the Hemmings home, which is already high. Byron's father Seymour, who works in the city and comes home only on weekends, is wildly jealous, suspicious of Diana, obsessed with appearances, and distant toward Byron and his sister Lucy. Thus Diana - who seems to have an "unrespectable" history and takes some kind of medication - is determined to keep the accident and new friendship a secret from her husband.
This story alternates with anecdotes about a man named Jim that take place forty years in the future. Jim - who has spent most of his life in mental institutions - is now out. He has a bad stammer and is severely handicapped by obsessive compulsive disorder. However, Jim is able to live in his camper and maintain a job cleaning tables at a supermarket cafe. Jim is almost incapable of interacting with other people but seems to want to befriend his co-worker Eileen.
The author does a good job conveying the ambiance of the Hemmings home as well as the mental/emotional states of the main characters. I don't want to give away spoilers so I won't say more except that the story provides an interesting treatise about obvious and not-so-obvious mental breakdowns.