Professor Sherman uses a giant balloon to travel over the Pacific. He lands on an island where he discovers a secret colony of people who also use balloons. Some are part of an amusement-park-type aerial ride, the Balloon Merry-Go-Round, while others serve as the lifting force for an escape life raft, should the island become too dangerous to live on.

The story’s balloons represent the soaring aspirations of humans: to invent clever devices that float over the Earth, to explore, to feel free from care. When they crash, the balloons also warn of the dangers of grand inventions and even bigger ambitions. They call to mind the ancient Greek myth of Icarus, who built wings of feather and wax that melted when he soared too close to the sun.

Twenty of the 21 balloons mentioned in the novel’s title—the 21st is Sherman’s own balloon, the Globe, with its basket-house gondola—are attached to a wide platform made of light wood. The balloons, folded and ready for use, are attached to hoses connected to large cylinders of compressed hydrogen. During the escape, these cylinders are opened, and the gas quickly fills the balloons, which lift the platform and its passengers away from the exploding island.