The Cisco Kid finds a million-dollar castle in a desert, a rich mine that doesn't exist and a vicious gunman. A lone bandit robs and kills a miner, knocks out Sheriff tom Roscoe, who is standing guard, and flees on Roscoe's horse. Almost immediately, Cisco and Pancho arrive on the scene. The Sheriff arrives and arrests them for the murder! As the Sheriff is taking Cisco and Pancho to jail, he's suddenly covered, disarmed and routed by an old "desert rat" named Grampus White, the "Arroyo Millionaire". Grampus takes Cisco to his "castle" a palace of mail-order luxuries in the middle of the desert. Grampus wants him to find his grandson Carl, who keeps Grampus supplied with gold dust from their secret mine, ( so secret even Grampus doesn't know where it is located). When Carl arrives, a month overdue, Cisco deduces that Carl's the bandit who shot and robbed the miner. The treasures in Grampus' "castle" were bought with stolen gold! Carl's "secret mine" is really his.44!