The evil wizard Khromm threatens a tribe of humans at the Alamo. They can protect themselves by using a sophisticated Guardian machine that utilizes flying robot drones that can disintegrate the wizard's laser tanks. But when it blows a circuit, the fight seems hopeless as replacement circuits have not been manufactured since before the Great Cataclysm. Thundarr and the group must use Khromm's Moon Dial to journey into the past to get the necessary replacement part for the Guardian. They arrive in 20th Century San Antonio, which is a much more peaceful and prosperous place than from whence they came. There they are aided by a little girl named Samantha, who shows them where to find the needed replacement circuit and they learn through her there was an earlier battle at the Alamo. They ride through town, obtaining the circuit from an electronics store, but the Moon Dial starts to close. The trio thank Samantha for her help, leaving her with a policeman just as they jump through the portal.
Returning to their time, Thundarr destroys the Moon Dial so Khromm couldn't use it anymore to travel to Old Earth to steal its weapons. Then they repair the Guardian by replacing the shorted out circuit with the one they got from the 20th century and the slave revolt is a success. With Ariel banishing Khromm after his army is defeated. Having realized the slave revolt was a Second Battle of the Alamo, Thundarr remarks that he and Ookla should pay more attention to Ariel's lessons, being grateful for a unique chance to see Old Earth.