Edred and Elfrieda Arden belong to an ancient noble family which has fallen on hard times. Orphaned and living with their aunt who takes in lodgers at the seaside, life is dull and frustrating. But when Edred suddenly inherits the title of Lord Arden - and a medieval ruined castle to go with it, the adventures are only just starting. The children discover a magic spell through which they meet the Mouldiwarp - a white mole who is also the emblem of the Ardens. They learn of a hidden treasure and discover powerful magic which allows them to travel back into the past to seek for it. One adventure follows another, as they encounter smugglers, highwaymen, plotters, prisoners in the tower, kings and witches... until finally they discover the treasure, which turns out not to be what they had expected - but much much better.