John is a young boy who loves chocolate more than anything else and refuses to eat healthy foods. Despite his intense greed regarding sweets, he generally cares about his friends and wants to do his best at school. His obsession leads him to a mysterious candy store where a strange coin buys him a box of chocolate. Eating the single piece inside grants him a magical ability that turns everything he puts in his mouth into chocolate.

The Storekeeper is a mysterious man who runs a hidden candy shop that appears on an empty lot. He accepts a strange coin bearing John's initials as payment for a large box of chocolate. He operates with supernatural knowledge about his customers and seems to exist specifically to provide life lessons about greed and personal responsibility.

Dr. Cranium is a tall, thin doctor with a bald head and a gray mustache. He prescribes tonics for John to replace the nutrients missing from his candy-heavy diet. He has a large ego and cares more about advancing his own medical reputation than actively finding cures for his patients.

Susan is a young girl with yellow curls, pink cheeks, and blue eyes. She is a friend of John's who experiences rapid emotional shifts when her prized possessions are damaged. Her reactions demonstrate how quickly childhood friendships fracture when tested by unexpected conflict.

John's parents are traditional authority figures with the father working and the mother keeping house. They worry constantly about John's nutrition and try to encourage him to think about how his actions affect others. They view his strange behavior simply as an extension of his intense candy obsession rather than anything supernatural.

Miss Plimsole is a strict schoolteacher who wears glasses and maintains order in her classroom. She focuses strictly on her teaching duties and ignores unusual events that do not fit into her job description.

Spider is a school bully who is one year older than John. He cares only about getting what he wants and shows no concern for how his actions affect others, aggressively taking what does not belong to him.

John's sister lives with the family and closely observes John's strange new habits. She attempts to report his unusual behavior to their mother, though her warnings are dismissed.