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 doings at the pond. Sherlock and Watsaon meet large nefarious beasts, and the scurrilous Moriarty Magpie.

Hermie, the caterpillar with a surprising trick, ancd two more of Sherlock's friends, Beatrice and Benedict, help to solve the mystery and make the poisoned pond safe again.

This is the third book in the popular Sherlock Ferret series, which introduces the world of detection to younger readers in a way that also delights any adults who read the books.