George's life is turned upside down when she learns that her cousins are coming to stay for the summer. But when Julian, Dick and Anne arrive, George finds herself including them in what turns out to be the most exciting adventure of her life so far.

When a body washes up on the beach, apparently the victim of a local curse, the cousins and Timmy the dog set sail to the source of the problem – an uninhabited island that belongs to the family.

In a subterranean chapel protected by a deadly booby trap they discover a goblet whose engraving strongly suggests that a lost hoard of Knights Templar treasure is waiting to be discovered. But the Five are not the only ones looking for treasure. Ruthless antiquarian Thomas Wentworth and his right-hand man Monsieur Boswell are also on the hunt, and they're one step ahead.

Will the kids reach the cursed treasure before Wentworth does? And if their nemesis is right about its true power – what then?