Otto, Martha and Scarlett Crumb are three most unusual children. With their dad, Henrig, busy inventing a special case to protect the Golden Elephant of Heera, they're expecting to look after themselves over the summer. Yet suddenly Henrig's long-lost sister turns up – and she's beastly! The children think things can't get any worse ... but then the Unstoppables, a notorious gang of robbers, set their sights on Henrig's special case, and it seems they'll stop at nothing to get their hands on it!