William Trundle loves dinosaurs. In fact, he's obsessed with them. Meanwhile, his father is equally obsessed - but with Christmas.

Aside from that, life is fairly normal in the Trundle household. William goes to school, has plenty of friends and is quite happy being a wheelchair-user. All that changes however, when Brenda Payne (the most rotten girl in the world) starts at his school and makes him feel different.

Meanwhile, in the North Pole, a rare dinosaur (indeed the last dinosaur alive) is in a similar position. The Christmasaurus loves his life, his elf friends and the delightfully jolly Santa Claus, but can't help but feel alone.

Sophie, an orphan, is taken away by a giant named BFG (Big Friendly Giant) as she sees him in the witching hour (a time when everyone is sleeping and giants show up). He takes her away because he is afraid that she will tell everyone and he will be in danger. BFG is a good giant. But his fellows aren't.