Three hours is 180 minutes or 10,800 seconds.

It is a morning's lessons, a dress rehearsal of Macbeth, a snowy trek through the woods.

It is an eternity waiting for news. Or a countdown to something terrible.

It is 180 minutes to discover who you will die for and what men will kill for.

In rural Somerset in the middle of a blizzard, the unthinkable happens: a school is under siege. Told from the point of view of the people at the heart of it, from the wounded headmaster in the library, unable to help his trapped pupils and staff, to teenage Hannah in love for the first time, to the parents gathering desperate for news, to the 16 year old Syrian refugee trying to rescue his little brother, to the police psychologist who must identify the gunmen, to the students taking refuge in the school theatre, all experience the most intense hours of their lives, where evil and terror are met by courage, love and redemption.