In 1862, Fergusson, an engineer, is commissioned by the British government to fly his latest invention, a giant balloon carrying a gondola, 4,000 miles from Zanzibar into uncharted West Africa to claim the land around the Volta River for England. Time is of the essence, since a band of international slave traders is already pushing overland to claim the territory. Fergusson is accompanied by his young assistant, Jacques. In Zanzibar they are joined by Donald O'Shay, an American playboy reporter who will write an eyewitness account of the journey, and by a runaway female slave, Makia. Sir Henry Vining, president of the Royal Geographic Institute and debunker of the invention, is sent as the Queen's envoy to accompany them. Several days later, they rescue an American mission teacher, Susan Gale, from the clutches of a bulbous, drunken sultan, and take her aboard. Also on hand is a leering, cowardly Arab slave trader, Ahmed. When the balloon is forced down by a sandstorm, all except O'Shay are captured as infidels and ordered thrown from the tallest minaret in Timbuktu; but O'Shay arrives with the balloon and plucks them to safety. When they finally reach the Volta, the balloon collapses into the river just as the slave traders are approaching. O'Shay safely plants the flag for England, and Ahmed routs the traders by stopping their leader with a well-aimed dagger. With their mission accomplished, the adventurers shake hands on a job well done.