After his father is captured in South America and presumed dead, Edred Arden inherits the title Lord of Arden and moves with his sister Elfrida into the family castle. Here the children discover an ancient spell, which they use to conjure up the magical Mouldiwarp. With his help, they set off in search of the lost Arden treasure. Soon a magical time-travelling adventure begins and the children find themselves in the middle of the Napoleonic wars and imprisoned in the tower for treason. But still no treasure. Wiser but slightly disappointed, the children return to the present to discover the greatest treasure of all waiting for them...their father.