The Kid is a scrawny member of the Credible Hulks baseball team. The Hulks only have The Professor's handmade ball to play with - a cobbled-together creation of a rock that he coated with melted down bicycle tires, olive oil, and some hair from his baby brother's head.
Until one day, Leg's little sister Tag Along shows up, bringing a real baseball. This ball belonged to Tag-Along's grandfather, the home run king of San Juan, who played in the big time Puerto Rican Leagues.
The very first time they use the real baseball in a match against the Spit Vipers though, disaster strikes! One of the Vipers gets a hit, and the ball goes straight into a sewer grating where the street used to be. The grating is narrow, and The Kid is the only kid small enough to go down into the sewer to retrieve the ball.
Once he enters the sewer, The Kid meets magical and menacing creatures who've been living below the streets. Some are helpful - and some are very, very hungry. He must use all his wits to evade a crew of famished sewer dwellers, to bring back Tag-Along's grandfather's baseball and avoid being eaten in the process.