The royal family are disturbed by the shocking news that there has been a break-in at the Tower of London - everything has been stolen, including the crown jewels. The adults say it's a human matter, but Lupo has an especially bad feeling about this particular crime. He and Kitty take matters into their own hands. Meanwhile, something is stirring in the Thames - a creature who has been hiding in the sewers for hundreds of years. A creature with revenge on its mind ...