In rural Somerset in the middle of a blizzard, the unthinkable happens: a school is under siege. Pupils and teachers barricade themselves into classrooms, the library, the theatre. The headmaster lies wounded in the library, unable to help his trapped students and staff.

Outside, a police psychiatrist must identify the gunmen, while parents gather desperate for news. In three intense hours, all must find the courage to stand up to evil and save the people they love.