Roddy is a high-society mouse living in a penthouse apartment in London when an encounter with Sid, a sewer rat, sends him tumbling down the toilet and into a world he has never known. Stranded in the sewers beneath the city, Roddy must adjust to life in Ratropolis, where he meets the resourceful Rita and crosses paths with the rodent-hating Toad and his loyal thugs, Spike and Whitey.