For Philip, Dinah, Lucy-Ann and Jack, the holiday in Cornwall is everything they'd hoped for - until they begin to realize that something very sinister is taking place on the mysterious Isle of Gloom. But they're not prepared for the dangerous adventure that awaits them in the abandoned copper mines and secret tunnels beneath the sea.
Lucy-Ann, Jack, Dinah, and Philip spend the holidays in a cottage near a strange, deserted castle, and find themselves in the midst of another adventure, involving a torch flashing at night and men at work in the depths of the castle.
All the children are aboard for a quiet cruise among the Greek islands. But when Jack's pet parrot breaks a birthday present, all the children are plunged into an exciting quest to find the lost treasure of the "Andra" with some ruthless villains hot on their trail.