1862 England is the setting for a bold but perilous dream of flight. The Jupiter, a manned balloon with a unicorn-shaped gondola, plummets from the skies during its maiden voyage. Ahmed, a slave trader, and Sir Henry Vining, the head of the Royal Geographic Society, scream in terror as the craft collapses, while the calm genius behind it, Fergusson, remains composed, planning a dramatic demonstration to showcase the balloon’s controls. On the instructor’s cue, Jacques powers the ascent with a pressure gauge that promises no loss of gas or ballast. The near-disaster leaves Sir Henry and his treasurer wary of further funding, and they walk away from the project in frustration. An American publisher is willing to back the venture only if his star reporter nephew, Donald O'Shay, can come aboard; O'Shay, famous in the press for his rambunctious playboy persona, is presented as a harmless, inoffensive presence to Fergusson, even though Randolph knows the truth about him.
The story widens its ambitions when Parliament intervenes. The Prime Minister tasks Fergusson with a mission of value: to outpace a convoy of slave traders racing toward uncharted lands near the Volta River in West Africa. The objective is to plant the British flag there before the slavers can claim the territory. Fergusson calculates that five weeks on the ground—powered by air—will suffice, and the plan is to embark with Donald O'Shay as a neutral witness. But the expedition grows more complex when Sir Henry Vining insists on accompanying the party, claiming expertise about Africa and demanding to be acknowledged as the "General." The Queen’s unexpected decision adds an extra layer of tension and purpose to the journey, while in Zanzibar the crew meets the local authorities and the Consul, who scrutinize every move.
During a stop at Zanzibar, Jacques spots Makia aiding a kidnapped slave girl and fighting off traders as they try to sell her. The group manages to free Makia, who slips away to join them, while the locals threaten to reclaim her. A wild chimpanzee named The Duchess accompanies the balloon as they continue their voyage, adding both charm and chaos to the expedition. Their arrival in the Arab city of Hezak triggers a citywide stir: a Muslim priest proclaims that O'Shay is the Moon God and that the balloon itself is the moon, turning them into instant celebrities. The crew dines with the Sultan, but danger closes in when Ahmed—a slave trader who will become a comic foil—enters with an offer to purchase a kidnapped American teacher, Susan Gale. When the moon rises on the horizon, the illusion shatters—mortals, not gods, stand before the Sultan—and the group flees with Susan aboard the Jupiter as it launches again. Ahmed clambers aboard, adding a new layer of antagonism to the journey.
Along the way, misadventures mount and blame shifts to Donald O'Shay for misjudgments that put the balloon in danger, from tangling with hostile natives to accidentally dropping its anchor. The party debates whether they need two American witnesses, and while Susan looks easier to manage, they decide to temporarily turn Donald O'Shay over to roaming Arab nomads. A looming sandstorm forces a dramatic reversal just as their path seems clear. Near Timbuktu, the balloon lands in an oasis where the group is captured by a mounted patrol of the Sheik of Timbuktu. Jacques, Ahmed, and Donald O'Shay work to improvise an escape, disguising themselves as they slip away in the balloon, which remains a source of fear for the patrol.
In Timbuktu, Fergusson, Susan Gale, Sir Henry Vining, and Makia are imprisoned as infidels and slated for execution from a tower. Makia’s warnings reach the group as Jacques, Ahmed, and O'Shay strike a bold plan: they purchase Makia from a trader, while Makia alerts them to the scheduled executions. The team pilots the Jupiter into the tower, fending off a wave of swordsmen and freeing their companions. As they ascend again, a swordsman’s strike slices the balloon’s support, threatening their chance to reach the Volta River and claim the land.
With the balloon leaking and heavy cargo weighing them down, [O'Shay] and Ahmed rally the crew to push through the night. They reach the Volta River first, but the wound from the scimitar persists; the balloon continues to lose altitude as the slave traders close in. To gain height and destroy a hanging bridge, the team ascends the crow’s nest and releases the gondola, sacrificing cargo to buy time. The explosion of effort sends the balloon spiraling toward water near the river’s edge and an approaching waterfall. The group swims to safety, while Ahmed, unable to swim, rides the balloon as a makeshift raft. O'Shay dives back for the British flag, while Ahmed carries the flag to safety and defeats the slave-trade leader with a dagger to the heart. The act earns a hard-won respect from the crew, and Sir Henry admits he misjudged Fergusson.
In the end, the expedition seals its fate with a warm reunion: Jacques and Makia share a moment of relief, Susan Gale and Donald O'Shay find renewed balance, and even the Duchess forms a surprising bond with a newfound chimpanzee friend. The Jupiter’s voyage—ragged, dangerous, and deeply unlikely—has carved a path across a wild continent, redefining courage, collaboration, and the stubborn human drive to conquer the unknown.
