Taking place in the future (then stated as "1994"), the Earth has experienced major changes due to the fallout of a stray asteroid. With most of the Earth in ruins, and the Moon now broken into two orbiting pieces, various rudimentary civilizations exist that are ruled by wizards and warlords. The plot concerns main hero, Thundarr [Robert Ridgely] - a strong yet unintelligent wielder of the Sun Sword. The sword was simply a hilt whose energy blade was activated on command by Thundarr, much like a Star Wars Lightsaber.

Along with companions Princess Ariel [Nellie Bellflower] - a powerful sorceress, and Ookla the Mok [Henry Corden] - a statuesque and strong ape-like beast. Each episode had a setting within a different metro area of the United States, featuring many popular landmarks including the Statue of Liberty (perhaps inspired by Planet of the Apes). The trio fended off evil wizards such as Gemini and Octagon, as well as a host of bandits, pirates and mutant creatures. On occasion, they use old Earth remnants to their advantage - including old military equipment.