An adventurous professor fights to keep his hot air balloon safe on a rescue mission.

In an old fortress on the Nile, a despotic Hindu is holding as his prisoner a British explorer, Sir Hubert, who has discovered the hidden tomb where Cleopatra's treasure is buried. Sir Hubert refuses to disclose the location of the jewels, and the Hindu goes to England and tricks the London Geographical Society into organizing a rescue mission. Included in the expedition is Sir Hubert's fiancée, Ellen, whom the Hindu plans to torture until Sir Hubert agrees to talk. A young explorer, Dr. Faraday, convinces the Society that the trip should be made by balloon, and he, Ellen and the Hindu set out for Egypt. After several adventures, including their temporary capture by cannibals and an attack by huge condors, the trio arrive at the Nile fortress. The Hindu has Ellen tortured on a stretching rack, but the greedy Sir Hubert still refuses to divulge his secret. They are both rescued by Faraday, but Sir Hubert dies trying to load the balloon with chests of treasure. Faraday and Ellen make their escape but are forced to jettison the gold and jewels in order to keep the balloon aloft. Ellen, however, saves one diamond for a wedding ring.