Scientist Dylan Hunt volunteers for an experiment deep in the Carlsbad Caverns, locking himself in a pressure chamber to test out a form of suspended animation. This will be used so astronauts can travel far in space. Soon after, there's an earthquake and the chamber is buried beneath tons of rock. He's awakened by a group of humans in the year 2133. Their society is called PAX. But Dylan is sick from his sleep until the woman, Lyra-a (Mariette Hartley) who is nursing him back to health, concocts an antidote from mutated plants She's the one who explains to him where he is and that PAX is a lie- that they want to conquer other societies and she was sent there, by her people, to spy on them and gather information. She also reveals to him that she's a mutant--with two navel and two hearts, superior in strength and intelligence to humans. At first Dylan believes her--and is suspicious of PAX. The leaders are worried what Lyra-a has told him and assign him a new nurse, Harper-Smythe, who is of the "Uni-sex" persuasion. Harper informs him that the downfall of his civilization was because of lust.
That night Lyra-a breaks into his chamber, ties up Harper, and breaks him out of PAX to take him to her city. It's a wilderness on the surface. The sub-shuttles, which connect cities--and even continents--are still working because they are nuclear powered. When they leave the first station they encounter a group of cavemen-like people who clearly fear--and obey--Lyra-a. Once at the city she tells her rulers about him and they say how they need to convince him that they are friends because they want him to repair their nearby nuclear power plant, which is going on the fritz. But Dylan is bothered by how all the humans are treated--like slaves--and he escapes his room to explore the area. He runs into members of PAX, including the giant Isiah (Ted Cassidy). They tell him the real story of PAX--that they are the descendants of peace-loving scientists and want to preserve art and knowledge. They also show him how "new helpers" are captured--and are indeed slaves. Their plan is to instill a revolt in the city's human population. Dylan is taken to the council and refuses to repair the nuclear power facility--so they use a "Stim" on him. This is a baton-like weapon that causes intense nerve pain. He's now treated like a slave.
PAX now wants their infiltrators to forget the human revolt and rescue Dylan instead--but Dylan wants to stay and help.
The revolt works and they all escape and race to the underground shuttle. There, they run into Lyra-a, who really does have strong feelings for Dylan. Dylan chooses to stay behind, to let the others escape. Six days later the council at PAX gets news that Lyra-a's people have found a viable nuclear warhead--and this is when Dylan returns. He has set a timer to explode their nuclear power facility and they all see this from PAX's above-ground entrance. Since they are peace-loving, they reprimand him for this--and tell him that if he's to join them he can no longer use violence as an option. He agrees.
I've always liked this movie--as well as the follow-up PLANET EARTH, which starred John Saxon in the Dylan Hunt role.