Three teenagers meet up by chance on a day of deep snow at a deserted castle in a remote part of East Anglia.

They are trespassing in the castle grounds in order to go sledging.

The three main characters are all at odds with their families for various reasons.

Emily, feisty and argumentative is simply bored. Simon comes from a large dysfunctional family. Marcus from a town some miles distant is imaginative and enthusiastic but has some strange and disturbing characteristics that suggest all is not what it seems with him.

The three of them manage to get inside the castle and decide to spend a night there.

They become drawn into its historical and ghostly past but in the morning they split up fractiously and return home.

However, Emily and Simon discover that Marcus has not gone home and is continuing to camp out.

When his father is spied lurking outside the grounds, they start to prepare the defences of the castle spurred on by Marcus's story that his father has beaten him up.

The police arrive and a full-scale siege develops. Beginning as a backdrop to a winter game, the castle becomes ever more alive to the teenagers and the siege becomes an actuality.

The final confrontation is