Joe can’t believe his luck when his dad rents a castle for their Easter holidays - but is devastated when his mum receives a better invitation and the trip is cancelled. Determined to have his holiday, Joe develops an elaborate plan to go with his friends instead.

The castle is everything the friends had hoped for: a partially restored ruin with a moat, a Saxon chapel, spiral staircases and an ancient burial mound that gives it an air of mystery. But it’s not long before they hear strange noises underground and notice yellow eyes lurking in the forest. And when Sherlock falls through some rotten boards and into an old well, they discover an ancient box, containing a 200-year-old notebook and an incomplete treasure map.

The race is on. The After School Detective Club is on the trail of Ragnar’s gold - a hoard of treasure said to have been buried by a Viking warrior and guarded by his demon hound … that is, unless someone else gets there first.