Set in 1993 London, About a Boy features two main protagonists: Will Freeman, a 36-year-old bachelor, and Marcus Blewer, an incongruous schoolboy described as 'introverted' by his suicidal mother, Fiona, despite his tendencies to bond and interact with people. Will's father wrote a successful Christmas song, the royalties of which have afforded Will the ability to remain voluntarily redundant throughout his life – he spends his huge amounts of free time immersing himself in 1990s culture, drinking, and pursuing sexual relations with women.
After a pleasant relationship with a single mother, Angie, Will comes up with the idea of attending a single parents group as a new way to pick up women. For this purpose, he invents a two-year-old son called Ned. Will then makes a number of acquaintances through his membership of the single parents group, two of which are Fiona and her son Marcus. Although their relationship is initially somewhat strained, they finally succeed in striking up a true friendship despite Will being largely uninterested during the early-middle stages of the novel. Will, a socially aware and "trendy" person, aids Marcus to fit into 1990s youth culture by encouraging him not to get his hair cut by his mother, buying him Adidas trainers, and introducing him to contemporary music such as Nirvana. Marcus and Will's friendship strengthens as the story progresses, even after Marcus and Fiona discover Will's lie about having a child.