The puddle in the road seemed like the perfect place for Tommy the mouse to test out his latest get-rich-quick scheme. However, just as he and his cousins had rowed the raft to the centre of the water, a passing vehicle splashed through it, barely missing them but creating a wave that shoved the raft and its passengers into an old drainage slot.
Having fallen down the drain, the companions find themselves at the mercy of the raging currents in the sewerage system. The raft is carried deeper and deeper through a confusing maze of passageways and grottos in which fire and a wisecracking piranha put the mice in constant peril.
Eventually the craft enters an old coal mine, where a bullfrog named Abner appears and tells the companions he knows of a route he can guide them along that will take them back up to the surface.
But with swashbuckling trouble chasing them and other dangers to be faced directly ahead, will the companions end their days in the sewer, or finally escape from below.