In 1700, when pirates are plundering merchant ships along the trade routes to India, Lieut. Brian Fleming, a British naval officer motivated by the reward money offered to check the piracy, poses as a tumbler in Zucco's acrobatic troupe in order to gain access to the buccaneers' stronghold in Madagascar. After demonstrating his courage and daring, he joins the pirates as navigator to first mate John Avery, even though the two men are competitors for the affections of Jessica, a fiery pirate girl who has inherited control of the Madagascar fortifications from her father. During their first mission together, they capture the state ship of the Emperor of India, and Brian takes as a hostage a young woman who turns out to be the Emperor's daughter, Princess Patma. When Patma is auctioned off, Brian tries to purchase her, but he is outbid by Jessica, who is determined to have Brian for herself. Eventually Avery discovers Brian's true identity and lashes him to a stake in the shark-infested surf. But Jessica rescues him as British war vessels move in to bombard the pirate fortress. When Avery attempts to escape aboard his ship, Brian and Zucco's acrobats stage a surprise attack, and Avery meets his death in a duel with Brian. Arrangements are made for Princess Patma to be safely delivered home, and Jessica, now betrothed to Brian, is granted a Royal pardon.