I would say that this book is mainly about loneliness, but also about relationships.
Marcus: Marcus, the oldest twelve-year-old in the world, looks after his suicidal depressive mother, listens to Mozart, and doesn’t really have any friends. His parents are divorced, and he has just moved from Cambridge to London with his mother. He doesn’t get along very well with his new schoolmates. He also feels responsible for his mother’s depression. In spite of all his problems both at home and at school, he’s still a boy with lots of courage.
Will: Will, a good-looking thirty-six-year-old “teenager”, is totally different from Marcus. He wears the right clothes, goes to the right clubs, reads the right magazines and listens to the right music. He doesn’t work, but lives from the royalties of his father’s song; “Santa’s Super Sleigh.” He’s single, childless, and is always out looking for a single mother to “score”. He doesn’t really have a lot of friends, so he feels kind of lonely sometimes.
Fiona: Fiona is Marcus` mother. She is a bit hippie-like, and has a great effect upon Marcus and his likes and dislikes. She suffers from depression, and cries almost all the time. She misses having a boyfriend and she’s tired of being left alone with the responsibility of Marcus.
They all meet when Will decides to join the single parent group (SPAT) to hook up with some lonely, single mothers. He hasn’t actually got a child of his own, but he solves this problem easily by inventing a two year old son, Ned. At his first SPAT meeting, Will meets Suzie, a single mother with a little girl. Suzie thinks Will’s got a son, so she invites them both to a SPAT picnic one Saturday. Ned can’t be there of course, simply because he doesn’t exist. So Will has to come up with an excuse to explain Ned’s absence.
At the Spat picnic, Will meets Marcus for the first time. Marcus is there because Suzie’s looking after him for his mother. (Will is there to “score” with Suzie.) Both Will and Marcus think the SPAT picnic is very daft. At first they are a little skeptical about each other, but this loosens up when Will decides to cover up for Marcus’ accident.
You see, Marcus is so bored that he accidentally kills a duck by throwing french loaves at it. When the park-keeper arrives, Will tells him that Marcus didn’t kill the duck, he just tried to sink the already dead duck-body with some bread.
Because of this, Marcus changes his opinion about Will, and thinks of him as quite cool after all.
After the SPAT picnic, Suzie drives Marcus home. (Will’s also in the car, because his “non-existing wife” apparently has taken his car along with everything else.) When they arrive at Marcus’ place, they find his mother, Fiona, in a pool of her own vomit. On the desk there’s an excuse note. At first they think she’s dead, but luckily she’s not.
They get her to the hospital, and she recovers quite fast. Marcus, however, doesn’t. Deep inside he remembers this as “the Dead Duck Day”, and the day when his mother tried to kill herself…
After “the Dead Duck Day”, Marcus starts turning up at Will’s apartment, and they develop a relationship neither of them have ever had before, where Will can give Marcus something his mother can’t. Marcus spends almost every afternoon at Will’s place, and Will gives him advice and buys him shoes more fit for his age.
When Fiona finds out about these after-school visits, she strides off to Will’s apartment, with the impression that he has been “fiddling” with Marcus. They have a huge discussion where lots of things come up, and Will tells Fiona about Marcus` situation at school, where he is being given a hard time by the other students.
After this incident, Marcus is no longer allowed to go to Will’s apartment for a while.
Will is invited to celebrate Christmas with Marcus, Fiona, Marcus` father, Clive, his girlfriend, Lindsey, and Lindseys mother. In spite of some minor disagreements concerning Wills imaginary son, this turns out to be one the best nights of both Marcus` and Wills lives.
On a new year – party, Will falls in love with Rachel. She starts talking to him, and somewhere in the conversation Rachel accidentally thinks Will has a son, just like herself. ( As Will sees this as an opportunity to get to know Rachel better, he doesn’t protest on this misunderstanding). Rachel invites them both to her place, and Will has to get Marcus to be his “son”.
After a few visits at Rachels house, Will feels like he has to tell her that Marcus is not his son. In the beginning she’s very crossed with him, but she forgives him, because although he is not Marcus` father, it’s obvious that he cares for him, almost like a father.
Marcus has met this girl, Ellie, who really fascinates him, and she “protects” him from the other students. She also teaches him about rock stars.
Over the months, Marcus gradually changes from being an behind-the-times, weird and shy boy, to being a boy who slides into the crowd with his normal clothes, taste of music and way of being.
Will has changed as well, from being a lonely and selfish “teenager” to being a more grown-up man with relationships and responsibilities.
I would say the message/ theme of the book is children of divorced parents, and how this is hard on both child and parent. Will enters as a kind of father-figure for Marcus. The message can also be the difficulties of growing up in a society where it’s hard to find who you are without following the crowd. This can often lead to loneliness, which ( as mentioned on top ) I think is the main subject in the novel.