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 circuits as no longer made in the wastelands of New Earth, 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 from whence they came. There they are aided by a little girl named Samantha, who helps them acquire 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 a hardware 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, they repair the Guardian and the slave revolt is a success, banishing Khromm. Having realized the slave revolt was a Second Battle of the Alamo, Thundarr remarks that he an Ookla should pay more attention to Ariel's lessons, being grateful for a unique chance to see Old Earth.