Will Freeman lives a carefree lifestyle without any responsibility or commitments, thanks to royalties left to him by his father's successful Christmas song. Will joins the Single Parents Alone Together (SPAT) community group under the pretense that he is the father to an imaginary child, but instead aims to meet single mothers. Will meets Suzie who he is attracted to but also meets Marcus, the socially awkward son of one of Suzie's friends unexpectedly on a planned play-date. When Will and Suzie take Marcus home, they find Marcus' mother Fiona (who suffers bouts of depression) has attempted suicide and rush to get her to the hospital in time.
Marcus soon discovers that Will's imaginary son does not exist, and uses it as leverage by blackmailing Will to go out with his mother as he wants to ensure Fiona doesn't get depressed again. Though Will shows no interest in Fiona, he begins to bond with Marcus and ultimately matures as an adult as a result. Will helps Marcus to establish confidence, but this plan soon backfires, and Marcus accidentally reveals that Will is not a parent. Fiona confronts Will and demands an explanation. Will retorts that he is only building confidence in the boy because he is routinely humiliated and bullied at school. Though Will promises to cease further contact with Marcus, Fiona recognizes that Will's genuine interest in her son is good for the both of them.
Will joins Marcus and Fiona for Christmas, gifting Marcus a Mystikal album as well as a portable CD player. During an exchange with Suzie who knows by now of Will's earlier deception, Marcus stands up for Will, and in turn, also stands up to Fiona's confrontational methods. Will defuses the situation, citing an earlier incident where Marcus accidentally killed a duck with his mother's cottage loaf. Marcus invites Will to Christmas dinner, and Will genuinely enjoys his day with Marcus' family. As Marcus develops his first crush at school (Ellie), Will also meets Rachel, who he hits it off with, and for the first time in his dating life, he begins to develop a serious interest in a woman. When Rachel begins to lose interest in Will as he describes his lifestyle, he brings up Marcus, and she responds as she is a single mother and believes Marcus is Will's son. Will fails to correct her, and asks Marcus to pose as his son for a while. Marcus eventually encourages Will to tell the truth, but when Will does so and admits the truth to Rachel, it does not have the effect Will had expected, and the relationship ends.
Fiona's depression returns, and Marcus goes to Will for help. Will, however, is still upset over his break-up and lashes out at Marcus. He soon finds his previous life of self-dependence unfulfilling, missing the company Marcus provided. Will decides to reconcile with Marcus and talks to Fiona about her depression, but then finds out from her that Marcus is due to perform at a school talent show that night, which would ultimately result in him being humiliated for the rest of his school life. As expected, Marcus' performance is greeted with brutal mockery, but Will joins in with a guitar, ultimately saving him from total humiliation.
By the next Christmas, Will and Rachel reconcile, with Will having abandoned his previous lifestyle permanently. Will invites one of his Amnesty International colleagues to set him up with Fiona, and Marcus and Ellie remain good friends.
Hugh Grant as Will Freeman
Nicholas Hoult as Marcus Brewer
Toni Collette as Fiona Brewer
Rachel Weisz as Rachel
Natalia Tena as Ellie
Sharon Small as Christine
Nicholas Hutchinson as John
Victoria Smurfit as Suzie
Isabel Brook as Angie
Jenny Galloway as Frances / SPAT
Augustus Prew as Ali
Denise Stephenson as Lindsey
Rosalind Knight as Lindsey's mum
Sir Tim Rice as himself
Jason Salkey as Amnesty International worker
an adaptation of the 1998 novel of the same name by Nick Hornby