Loonette and Molly spend the day discovering light and heavy things. After reading a story about a balloon, Loonette finds a balloon of her own, which she thinks can lift her off the ground. She's too heavy, of course, but the balloon does lift Molly off the couch and into the garden.

Molly's birthday is approaching. She and Loonette hold a birthday party with games and costumes. They also have fun with big things compared to little things, particularly with a story about a gigantic cupcake.

The show opens with a flashback: Loonette and Molly can't sleep. They venture around their house during the dark before Loonette reads her about a Sleep Train's magical adventure. The next day, Loonette discovers Major Bedhead asleep on the job.