A mysterious plague, the Purple Death, ravages the earth. Dr. Zarkov, investigating in his spaceship, finds a ship from planet Mongo seeding the atmosphere with dust. Sure enough, Ming the Merciless is up to his old tricks. So it's back to Mongo for Flash, Dale, and Zarkov.
Dick Tracy's foe for this serial is the crime boss and Masked Mystery Villain The Spider/The Lame One and his Spider Ring. In the process of various crimes, including using his Flying wing and sound weapon to destroy the Bay Bridge in San Francisco and stealing an experimental "Speed Plane", the Spider captures Dick Tracy's brother, Gordon. The Spider's minion, Dr. Moloch, performs a brain operation on Gordon Tracy to turn him evil, making him secretly part of the Spider Ring and so turning brother against brother.
An imprisoned gunfighter must scatter to elude the authorities. Outlaws Bob Ford (Robert Vaughn) and Vic Rodell (Stephen McNally) are nabbed, but the governor offers them amnesty in exchange for their help in bringing Jesse and his brother Frank (Douglas Kennedy) to justice. Peggie Castle and Barton MacLane also star in this tale of the Old West's most famous traitor.
