Will is 36, and is the wealthiest, trendiest, but also the most selfish and lazy man in the world. He lives off the royalties from his late father's single hit, "Santa's Super Sleigh".
To try and meet some women who are desperate for sex, he joins a single parents' group called "SPAT", while pretending he has a son. There he meets Fiona, who is an eccentric, vegetarian hippy, whose son Marcus is 12, and a total misfit: gets bullied at school, has a stupid haircut and clothes (chosen by his mum). One day he and Will meet on an outing, but don't like each other very much.
Marcus sometimes sings in the classroom without realising it, which causes him to suffer even more. He also spies on Will during the day, and sees that he doesn't have a kid at all. He tries to make Will be friends with his depressive mother, who has now started crying frequently.
One day, the "dead duck day": Will and Marcus go to the park with another "SPAT" mother, and Marcus accidentally kills a duck (by throwing an entire loaf of bread at it). They return to his house, to discover that his mother has tried to kill herself. She is rushed to hospital, and recovers.
Marcus now starts visiting Will at his home regularly. Will gives him advice on "trendiness", and buys him some expensive trainers ("sneakers" in the US!). The very next day, these are stolen at school, and Marcus has to explain to his mother how he and Will have become friends, and how Will is trying to save him from the bullies at school.
Later on, Marcus becomes friends with a rough girl at school (Ellie), while Will finds a new girlfriend, who assumes Marcus is his son. Will tries to persuade Marcus to pretend he is, but Marcus won't hear of it.
Fiona has started crying again. Marcus tries to get help from Will, who refuses, now his old selfish self. Marcus remembers that his mum loves to hear him sing, so he joins a school event, where he will give a solo rendition of "Killing me Softly", which is bound to make him very unpopular!
As soon as Will finds out Marcus intends to perform this, he races to stop him, knowing that he will die of embarrassment. He arrives just in time, but Marcus defiantly performs, singing too high, out of tune etc, and gets booed loudly, until Will comes onto the stage, playing the guitar to accompany him. Will gets a bit carried away, and carries on playing and singing loudly, until an apple is thrown at him.