Although young Montague Mad-Rat lives in--or rather, under--New York City, he know very few rats besides his mother, who makes hats, his father, who builds mud castles, and his globe-trotting Aunt Elizabeth. But Montague's life takes an abrupt turn for the eventful the stormy day he meets Isabel Moberly-Rat on his way home from Central Park.

Home, for Montague, is an old sewer pipe. He now learns that there is a cityful of other rats out there who inhabit abandoned piers and lead considerably less eccentric and more luxurious lives than his family. What's more, these rats are in the...