John Midas loves candy, especially chocolate. Concerned about his unhealthy diet, his parents take him to the doctor, who prescribes nutritional supplements, but John continues to crave sweets. When he finds a strange coin – emblazoned with the initials J.M. – lying on the pavement, he wastes no time in exchanging it for a box of chocolates from a mysterious sweet shop that John has never seen before. When John gets home, he is disappointed to find that the large and lavish box contains only a single chocolate, but he devours it greedily, finding it "the most chocolaty chocolate he had ever encountered" (p. 34).
In the morning, John gets ready for school. He brushes his teeth, delighted at the chocolate flavoured toothpaste that he assumes his mother has purchased. At breakfast, his orange juice tastes like chocolate too, and his fried egg is a delicious combination of runny chocolate yolk and white chocolate. His mother is pleased that he is eating healthy food at last, but Mary, his sister, is upset by his gloating and teasing – her breakfast tastes the same as it always does. John goes off to school, marvelling secretly at his strange new power. As he ruminates he chews on his glove, which melts into a sticky brown mess. A bully, Spider, sees him eating his glove and demands a piece, but is humiliated when he bits into a piece of nothing but sweaty old leather. John is enjoying himself, but starting to get thirsty. He gets no relief from the water fountain, and is mortified when his pencil, his cafeteria cutlery, and finally his trumpet, turn to chocolate as soon as they touch his lips. He quarrels with his friend Susan, and ruins her birthday party when he tries to join in the game of apple bobbing. All the while he is feeling more sick and sorry for himself, and what initially seemed like a wonderful gift has become a horrible and embarrassing affliction. John tells his father what has happened, and together they seek out the candy store, but in its place find only an empty lot. Instead, they return to Dr. Cranium, who ignores John’s insistent claims that everything he touches turns to chocolate, and instead dispenses another dose of his patented elixir, which, along with the measuring spoon, promptly turns to chocolate. The doctor is amazed and excited, inventing the diagnosis of chocolatitis – or Cranium’s Disease – on the spot. He has ambitions for studying John’s condition in detail, but Mr Midas manages to extricate them from his surgery. Back at home, John’s mother is upset to learn of her son’s illness, and seeking to comfort her, John kisses her cheek, thus turning her into a chocolate statue. Terrified and remorseful, he flees from the house and manages to find the sweet shop once again. He implores the shopkeeper to remove the curse, promising to be less selfish and more truthful, and renounce his gluttonous ways. Reassured, John returns home to find everything back to the way it was. His mother, restored to her normal form and with no memory of what has happened, offers him a cold glass of milk, which he drinks with relish. Rejoicing that the curse has been lifted, John slips out of the house again to return to the shop, but finds only the abandoned lot, full of rubbish and a sign reading Sold.