Billy is a portly, pug-nosed young boy with a taste for adventure and risk. Desperate to buy a used minibike, he accepts a fifty-dollar wager to eat fifteen night crawler earthworms in fifteen days. He struggles with his fondness for junk food and his initial disgust for the wiggly creatures, using heavy layers of ketchup and mustard to mask the taste. He approaches problems methodically, using careful planning to outsmart his friends.

Alan is a scruffy, red-haired boy who tends to chew his thumb when nervous. He bets fifty dollars from his savings account against Billy, quickly realizing he will be in serious trouble with his father if he loses the money. This fear drives him to panic, leading him to invent increasingly desperate schemes to prevent Billy from finishing the contest.

Tom is a tall, skinny boy who agrees to act as Billy's "second" in the wager. He takes charge of cooking the earthworms in an old horse barn, trying different methods like boiling and frying in cornmeal. Though he coaches Billy to be brave, Tom secretly harbors an intense fear of consuming bugs himself.

Joe is the smartest and most educated boy in the neighborhood, possessing a lawyer-like mind for arguments and technicalities. He acts as Alan's assistant, devising clever rules, forging documents, and telling tall tales to frighten Billy into quitting the bet. Unlike Alan, Joe remains relatively calm because his own money is not at risk.

Billy's mother is a caring parent who initially worries about the health effects of consuming night crawlers. Once reassured by a poison control center that earthworms are harmless, she supports her son's unusual dietary project. She even uses a French cookbook to prepare a fancy gourmet bug dish to help him win.

Billy's father is a cheerful, sensible man with a good-natured sense of humor. He provides a stable, grounded presence in the household. He easily sees through the boys' exaggerated stories and forged letters, stepping in only when their competitive stunts become physically dangerous.

Mr. Phelps is Alan's father. While Alan lives in terror of his father discovering the unauthorized fifty-dollar wager, Mr. Phelps proves to be a level-headed man who values fairness. He expects the boys to resolve their disputes peacefully and demands apologies when their games disturb the neighborhood.

Pete is Tom's younger brother. He owns a loud portable siren and willingly tags along to help Tom and Billy execute late-night stunts to ensure Billy meets his strict daily deadlines.

Emily is Billy's younger sister. She observes the strange happenings around the house as her family adapts to Billy's daily eating habits, occasionally delivering the mail that contains tricks from his rivals.