Peter, Janet, Jack, Colin, George, Pam, Barbara, and Scamper the dog are enjoying a thrilling game of "Red Indians" in the dense, tangled woods of Little Thicket. But when Colin climbs a tall oak tree to hide from the others, he spots something absolutely shocking: a desperate man scaling the high stone wall of Milton Manor and escaping with a priceless pearl necklace!

The Secret Seven immediately call an emergency meeting in their garden shed headquarters to investigate the crime. Their hunt for clues leads them to a muddy footprint, suspicious tire tracks, and straight into the colorful, bustling camp of a traveling circus! But snooping around the circus tents is incredibly dangerous, especially when the thieves realize the children are on their trail. Packed with midnight stakeouts, clever disguises, and a terrifying encounter near the lion cages, the Secret Seven must use all their detective skills to find the missing pearls before the criminals get away!