When Mrs. Fieldmouse visits Sherlock Ferret and his friend Watson Mouse M.D. and tells them that her twelve children have all fallen sick, Sherlock has to find out what has been going on. Together with Doctor Solomon Sloth, he discovers the cause of their illness. Then Lestrade, a rhinoceros friend (though not a very big one) tells him of mysterious happenings at the pond. Sherlock and Watson meet large nefarious beasts, and the scurrilous Moriarty Magpie. Hermie, the caterpillar with a surprising trick, and two more of Sherlock's friends, Beatrice and Benedict, help to solve the mystery, and make the poisoned pond safe again.