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.