The Secret Seven are absolutely furious when their beloved garden shed headquarters is taken over by the gardener for a massive spring cleaning and painting project. Suddenly homeless, Peter, Janet, Jack, Colin, George, Pam, Barbara, and Scamper the dog are forced to hunt for a brand-new meeting place. They soon discover a fantastic, perfectly hidden sand cave nestled deep inside an old local quarry. After hauling over colorful cushions, ginger beer, and their favorite books, they transform it into the ultimate secret hideout.

But the excitement quickly turns to fear when they arrive one afternoon to find their belongings rummaged through and a strange, weathered notebook left behind in the sand! The diary is filled with cryptic, coded notes and a suspicious list of locations. When the Seven realize that a ruthless gang of thieves is using their new cave as a stash for stolen goods, the chase is on! Battling the annoying interference of Jack’s sister Susie—who has formed her own rival detective club—the children must decode the notebook, execute a daring midnight stakeout at the quarry, and trap the dangerous criminals before they strike again!