Ernie Banks, named for the legendary Chicago Cubs shortstop, is a troubled, 13-year-old juvenile delinquent. Abandoned on the doorstep of the Lakeside Home for Boys when he was three years old, his only proof that he once belonged to somebody is a vintage Ernie Banks baseball card, a crystal acorn he wears on a string around his neck, and a strange spiral birthmark on the bottom of his right foot.
As a last reprieve before being sent to a juvenile detention facility, Ernie is allowed to spend three weeks on a working farm. When Ernie arrives at the home of Russ Frazier, he learns that the widower's baby was kidnapped years before. Ernie is determined to solve the case. He teams up with Joey, a local tomboy, to investigate clues that lead them on a dangerous journey into a forbidden world of dark secrets, magic puddles, and the cavernous underground kingdom of the Puddlejumpers: eleven-inch-tall water creatures with whom Ernie has a mysterious connection.