Kent wants the Allen ranch. So he has Steve and his men rustle their cattle using Pete as an informant. When the Trigger Pals Lucky, Stormy, and Fuzzy fight back, Kent frames Stormy for the rustling.

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.