Mr Mogg accidentally kidnaps the king in his flying giant teapot, beginning an adventure among strange lands and stranger creatures; but will they get home?
Nelly and Tom go to spend a few weeks of the summer holiday with the explorer Mowbray Mogg. His house is filled with treasures from his adventures, including a giant teapot. When Mr Mogg decides to return the teapot to his giant friend Alcibiades, he builds an enormous balloon so that he can it fly back to the Land of the Giants.
Nelly and Tom persuade Mogg to take them on a test flight to the capital. There King Augustulus the Stupendous and his Herald inspect the balloon and teapot. Due to the king’s tampering the balloon is accidentally launched, carrying them high above the clouds and far out to sea.
They discover a new land, which the king names Stupendia, but learn it is peopled by monsters - monsters with teapots for heads! The teapot-heads put Mogg on trial and he and his friends barely escape with their lives.
They reach the Land of the Giants to find that Alcibiades is the last giant left. After a tragic accident Mogg and the children return home, only to face the accusation that Mogg planned to kidnap the king all along. Only a last-minute intervention saves Mogg from a second trial.