The story begins in 1865, during the Civil War, when Captain Cyrus Harding, prisoner of the Confederates, escapes in an enemy balloon with war correspondent Gideon Spilett; Jack Pencroft, a sailor; Herbert "Bert" Brown, Pencroft's adopted son, and Neb, Harding's black servant. A sailor with an adopted son and a servant in jail with his master is rather mysterious in itself. Hey, move on, it's a Katzman serial from Columbia. The balloon drifts in space for days and finally lands on a desert island. Also landing, in a better aircraft, is Rulu, a visitor from Mercury who seeks a radio-active material that will enable her to manufacture an explosive that will destroy the world or, at least, the portion known as Earth. Since it is in the 1860's and she has a spacecraft that will make the journey from Mercury to Earth, and all of the earthlings are riding horses or walking, her problem with Earth is none too clear, other than possibly hacked about not winning the Miss America contest. Harding and his hardy crew fight 15 chapters of battles against the island's natives, some passing-by pirates led by reliable Gene Roth, and Rulu and her Mercury henchmen, wearing the same headgear that "The Spider" wore in two previous Columbia serials, indicating the costume department went overboard on black head-coverings with spider webs on them earlier in the decade. Harding and his men are frequently aided by Captain Nemo, a man of mystery in charge of the only submarine (the Nautilus) around at the time other than the "CSS Hunley." Rulu, who also has the power to put grown men into a trance and does so a couple of times, including the native chief and all of his warriors, finally has enough of all the problems, but has gathered up enough "radio-active" material to at least destroy the island and does so.