It is 1865 and Union prisoners use a military balloon to escape a Southern prison camp near the end of the Civil War The balloon drifts for days and finally lands on a mysterious volcanic island with very unusual inhabitants Also landing in a better aircraft is Rulu a visitor from Mercury She seeks a radioactive material that will enable her to manufacture an explosive that will destroy the world or at least the portion known as Earth in this 15 Chapter Serial from the 1950s