When Elmo discovers a book called Once Upon a Monster in the library and brings it back to Sesame Street, he and Cookie Monster are magically pulled inside its pages. What they find isn't quite what Cookie remembered—the monsters in the story all have problems that need solving. Over six chapters, Elmo and Cookie help Marco throw a real birthday party in the Electric Jungle, restore Shelby's ruined garden, build Seamus's confidence for his play, befriend the Puffalopes for the frightening Grrhoof, help Tallulah's band get their act together, and inspire the storyteller Ramona with adventures of their own. It's a day of helping, dancing, and very, very good cookies.