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 midst of a grave crises. A human being has decided to turn their piers into parking lots, and an extermination campaign is already under way.

As Montague stumbles into this wider, bewildering world, he long to help ratdom (and impress Isabel). But what can he do, when his only talent is painting the seashells his Aunt Elizabeth brings him from her travels? And to make matters worse, it turns out that a drunken uncle of his, Montague Mad-Rat the Elder, has made their name a standing joke in the rat world. For the first time in his life, young Montague finds out what it is to feel helpless and alone--little realizing that he has not only Isabel and his seashells on his side but his despised drunken uncle as well.