Will, an immature, self-absorbed, thirty-eight-year-old bachelor, lives in a well-appointed London flat, surrounded by all the latest gadgetry. Will has never worked and lives off the royalties of a popular Christmas song, "Santa's Super Sleigh," the only hit of his late, songwriter father. Although he enjoys seducing many women, Will is incapable of forming any meaningful relationships and fears commitment and parenthood. After his married friend Christine sets him up with one of her co-workers, Angie, the mother of a three-year- old boy, Will begins another temporary relationship. He is surprised, however, when, in a reversal of his normal routine, Angie thinks that Will is sensitive and supportive. After announcing that he is too good for her, she breaks up with him to attempt a reconciliation with her husband. Will then gleefully realizes that there must be many other single mothers who are not looking for a long-term commitment, and after locating a self-help group called SPAT, Single Parents Alone Together, attends one of their meetings, posing as the single father of a two-year-old son. Suzie, one of the mothers, shows an interest in Will, and they arrange to go on a picnic in Regent's Park. Faced with having to create evidence of his caring for a child, Will quickly buys an infant’s car seat for his imaginary boy. On the day of the picnic, Will explains to Suzie that his ex-wife has suddenly picked up his son for a visit. Suzie brings along the son of her best friend Fiona, twelve-year-old Marcus, who is not impressed by Will. However, when Marcus accidentally kills one of the park’s ducks with Fiona's stone-hard loaf of homemade bread, Will comes to his defense against the park keeper, and Marcus begins to look at him differently. When Will and Suzie drive Marcus home they discover that Fiona, a vegetarian hippie suffering from chronic bouts of depression, has attempted suicide. Later, after Fiona recovers and returns home, she resumes her tentative relationship with her son, who suffers at school due to his unconventional upbringing and nerdy appearance. Marcus, feeling that he cannot cope with his mother's problems by himself, decides he needs a back-up person in his family and phones Will to arrange for the three of them to meet in a restaurant. Will is not remotely interested in Fiona, nor is she interested in him, and the meeting goes badly. Over several days, Marcus follows Will during his aimless, self-indulgent excursions, then goes to his flat, accuses him of not having a son and offers not to tell anyone if Will agrees to date his mother. Will declines but Marcus does not give up and begins, without telling his mother, to visit Will every afternoon after school to watch Countdown , Will’s favorite television quiz show. Will begins to soften toward the boy when he learns that he is bullied at school because of his hair and out-of-fashion clothes, and takes him shopping for a pair of stylish sneakers. After paying for the shoes, Will realizes that he feels good about making Marcus happy. When the sneakers are stolen at school and Marcus arrives home shoeless on a stormy day, he is forced to tell Fiona about his visits with Will and that Will is not a father. Fearing the worst about Will's interest in her son, Fiona confronts Will in a restaurant, but he responds by criticizing her for not being aware of the torment Marcus is enduring in school. Fiona suddenly realizes that she has been misguided and asks Will to continue to see Marcus, but he is reluctant. Will, however, does accept Marcus’ invitation to join them for a Christmas dinner at their house, where he meets Marcus’ father and his current girl friend. Unfortunately for Will, Suzie is also invited and is angry with Will for lying about being a parent. To Will’s surprise, Marcus defends him, telling his mother that Will understands what kids need. At a New Year’s Eve party, Will meets Rachel, another single mother with a boy about Marcus’ age, and Will finds himself allowing her to believe that the boy he is speaking about is his son. Because Will is very attracted to Rachel, he is forced to enlist Marcus’ help in sustaining the deception when he visits Rachel at her home. Ali, Rachel’s maladjusted son, is in the same year at school as Marcus and, when they are alone, menacingly warns Marcus that he does not want his “father” becoming involved with his mother. Marcus suggests to Will that he should tell Rachel the truth about their relationship, and some time later, over dinner, Will begins by telling her that he is not Marcus’ natural father, then digs himself into a deeper hole until he is forced to confess the whole scheme and admit to being a “nothing.” One day, after Marcus finds his mother crying again, he assumes it is a prelude to another suicide attempt and decides to try to cheer and please her by signing up to perform her favorite song, “Killing Me Softly with His Song,” at a school rock concert. Marcus also asks Will to aid his mother, forcing Will, who is depressed over losing Rachel, the only woman he has ever cared about, to admit to him that he is unable to help in any meaningful way. Will begins to realize that his life is shallow and that knowing Marcus is the only thing that means something to him. Will then goes to another SPAT meeting and admits to all the mothers that he attended simply to meet women, whom he thought he could easily seduce. When Fiona tells Will that Marcus is singing in the school concert, Will realizes that the boy, who has no singing talent, will be further humiliated by performing the 1970s ballad in front of his rock- and rap-loving peers, and they both rush to the school to stop him. Even though the naïve Fiona feels that Marcus is simply expressing himself, Will convinces Fiona that Marcus is not expressing himself, but her. Meanwhile, the boy who was to accompany Marcus on a recorder decides not to subject himself to the embarrassment and drops out. When Will and Fiona reach the school, Will hurries backstage and tries to persuade Marcus not to perform, but he insists. Marcus is booed and heckled by his schoolmates, except Ellie, a popular girl on whom Marcus has a crush. Then Will suddenly appears from the wings with a guitar and plays and sings along with Marcus, eliciting a good response from the audience and saving the boy from social suicide. He also impresses Rachel, who is in the audience to watch Ali's performance. After the concert, Fiona thanks Marcus for the song and promises him that she will always be around. A few months later, Will hosts a Christmas dinner at his flat, with Fiona, Marcus, Rachel, Ali, Ellie and Tom, an Amnesty International worker who Will thinks would be perfect for Fiona. They have become a mutually supportive group of parents and children, and Will is considering asking Rachel to marry him. Thanks to Marcus, his twelve-year-old friend, Will has finally matured and come to realize, late in life, the value of friendship and commitment.