A tribe of humans at the Alamo protect themselves from the evil wizard Khromm by using a sophisticated Guardian machine that utilizes flying Robot Drones that can disintegrate the wizard's laser tanks. Once it blew a circuit, and Thundarr and his group used Khromm's Moon Dial to journey into the past to get the necessary replacement part for the Guardian as replacement circuits have not been manufactured since before the Great Cataclysm. After some troubles in 20th Century San Antonio, the team was able to obtain the circuit from the Guardian in the past.
Returning to their time, 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. (Portal Into Time)