Will trades his lack of enthusiasm toward children for a date with a truly beautiful woman who is also a single mother.

Nick Hornby's second bestselling novel is about sex, manliness and fatherhood. Will is thirty-six, comfortable and child-free. And he's discovered a brilliant new way of meeting women - through single-parent groups. Marcus is twelve and a little bitnerdish: he's got the kind of mother who made him listen to Joni Mitchell rather than Nirvana. Perhaps they can help each other out a little bit, and both can start to act their age.

Set in 1993 London, About a Boy features two main protagonists: Will Freeman, a 36-year-old bachelor, and Marcus Brewer, a 12-year-old 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 plentiful free time immersing himself in 1990s culture, music, and pursuing sexual relations with women.

After a pleasant relationship with a single mother of two, 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.

Marcus is befriended by Ellie McCrae, a tough, moody 15-year-old girl, who is constantly in trouble at school because she insists on wearing a Kurt Cobain jumper. He also spends some time with his dad Clive, who visits Marcus and Fiona for Christmas together with his new girlfriend Lindsey and her mother. Clive has a minor accident during some D.I.Y. work, and breaks his collarbone. This prompts Clive into having "a big think" about the meaning of his life, and he summons Marcus to Cambridge to see him. Marcus decides to bring Ellie along with him for support; however, they are arrested on the way as Ellie smashes a shop window displaying a cardboard cut-out of Kurt Cobain – accusing the shopkeeper of "trying to make money out of him" after his suicide.

Meanwhile, to Will's despair, he falls in love with a woman called Rachel. Rachel is a single mother with a son named Ali (Alistair), who is the same age as Marcus. The two originally fight, but quickly become friends. Will's emotional faculties are liberated and he begins to "shed [his] old skin" of emotional indifference; simultaneously Marcus is becoming more typical of his age, and he begins to enjoy his life more.

The title is a reference to the song "About a Girl" by Nirvana. This was confirmed by the author in the 2 December 2001 edition of the BBC Radio 4 series "Book Club".[2] The band is also mentioned several times in the book.