In 1987, NASA astronaut Captain William "Buck" Rogers (Gil Gerard) is piloting the space shuttle Ranger 3 when he flies into an unexpected space phenomenon and is frozen for 504 years. In the year 2491, his shuttle is found drifting in space by the alien flagship Draconia, which is headed to Earth for a trade conference, under the command of Princess Ardala (Pamela Hensley) and her aide-de-camp, Kane (Henry Silva), a former native of Earth. Rogers is revived from his cryogenic sleep. Princess Ardala is visible attracted to Buck, but Buck rebuffs the Princess and is put back on his shuttle and sent towards Earth.
It turns out that the Draconians are actually planning to conquer Earth through staged pirate attacks on Earth's shipping fleet, forcing Earth to seek a treaty with the Draconians and unwittingly opening up their defenses to the invaders. They plant a homing beacon aboard Rogers' shuttle to track a way to past Earth's planet-wide defense shield. Buck lands in New Chicago and is immediately taken into custody by Colonel Wilma Deering (Erin Gray), commander of Earth's military forces. He is interrogated and learns that Earth has been rebuilt over the centuries in his absence following a nuclear holocaust, and now the only thing left is this huge city surrounded by desert wasteland. Over the course of his time in detention, Buck makes the acquaintance of Dr. Elias Huer (Tim O'Connor) the leader of Earth's Defense Directorate, the AI computer Dr. Theopolis, and the robot drone Twiki (voice of Mel Blanc).
While recounting his encounter with the Draconians, Buck notices several discrepancies and suspects that the Draconians must be armed, contrary to the terms of the trade meeting. Against advice, Buck ventures outside the city to the ruins of old Chicago in an attempt to find out that what he has been told is real, eventually finding his own parents' grave and having to be rescued by Wilma and her troops from the mutants inhabiting the ruins. Following Buck's return to the Inner City, the Draconian tracking device is found aboard his shuttle, and the authorities accuse Buck of espionage. Buck claims that the Draconians set him up, and eventually Wilma persuades Dr. Huer to test Buck's claims by requesting a meeting with Princess Ardala and Kane aboard the Draconia. The Draconian marauders attack their flagship as a diversion, but Buck manages to destroy them single-handedly, thus earning Wilma's respect.
At the official diplomatic reception, Ardala, still attracted to Buck, invites him back to the Draconia, but Buck merely goes along to find out the truth behind the Draconians. On the flagship, Ardala says she needs "a man, a real man" to rule by her side and offers Buck the position. After drugging Ardala, Buck explores the flagship and discovers their plans to launch an imminent attack on Earth. Dr. Theopolis and Twiki, who have followed Buck aboard, eventually meet up with him and alert to the Draconian invasion.
Wilma immediately scrambles Earth's starfighters and attacks the Draconia, while Buck sabotages the Draconian marauders prepared to attack Earth and fights off Ardala's bodyguard Tigerman. During the battle, the Draconia is critically damaged, but Buck, Theopolis and Twiki are rescued by Wilma before the ship explodes. Ardala and Kane also escape before the Draconia's destruction in a shuttlecraft, while Kane vows to return and to take his revenge on Rogers.