It is 1862 in England. The Jupiter, a manned balloon with a unicorn-shaped gondola, falls from the sky during its maiden flight. Passenger Sir Henry Vining (Richard Haydn) and his treasurer (Ronald Long) scream in horror. However, Professor Fergusson (Cedric Hardwicke), the balloon's inventor, remains calm, as he planned on giving a dramatic demonstration showing the balloon's controls. On his signal, pilot Jacques (Fabian), ascends the balloon using a pressure gauge that ensures no loss of gas or ballast. Traumatized by their "near-disaster", Sir Henry, head of the Royal Geographic Society, and his treasurer refuse to fund Jupiter's exploration of East Africa, and walk out on the professor after landing. American publisher Cornelius Randolph comes to the rescue: He will back the venture if his star reporter and nephew, Donald O'Shay (Red Buttons), joins the crew. Unbeknownst to the professor, who is told by Randolph that O'Shay is an "inoffensive young man", O'Shay is notorious in the press for his troublesome antics as a playboy. On the day Fergusson intends to set sail for Africa, he learns that his expedition is halted and that plans have been changed. At the British Parliament, the prime minister commissions Fergusson to defeat a convoy of slave traders heading toward uncharted land near the Volta River in West Africa. The slavers aim to stake their claim within six weeks and take over the territory. Fergusson calculates he needs only five weeks to cross Africa by air and plant the British flag at the river. The Prime Minister recommends that he take O'Shay along as a neutral witness to the planting of their flag. However, he did not calculate the Queen sending along Sir Henry, who proclaims himself to be the "expert on Africa" and demands to be called the "General". While Fergusson meets with the British Consul in Zanzibar, Jacques spots O'Shay helping to free slave girl Makia (Barbara Luna), fighting off traders and disrupting her sale. When local authoriti