A group of four scientists (all right, three scientists and a comic sidekick) develop a technology that creates a time portal window. The klutzy sidekick guy (of course) jumps/falls right through and gets stuck in the far future, so the rest of them have to follow. The portal closes, trapping them in the far-flung year of 2071.

They find themselves in a blasted landscape and are immediately chased by "mutants," but are rescued in the nick of time by the last remaining true humans, descendants of those who survived intact from the atomic wars. They live in an underground bunker and have been frantically searching the cosmos for another habitable planet, because our solar system won't cut it anymore. They've built a spaceship, see, and it's only a matter of time before the mutants breach their defenses and destroy them all. They have to leave before those defenses fail.

The middle of the film is just an endless "gee whiz" of "future tech." Oh, and I should mention that someone--director, writer, whoever--must have been a frustrated stage magician, because there are many, many practical effects of "future tech" that are just rehashed stage illusions.

Sadly, however, our four time travelers ultimately can't go with the future-people because of weight/resource issues, so they have to try to re-create their time-window experiment. They succeed JUST as the mutants attack and destroy the spaceship and all but a handful of future-people, who escape to the past with our protagonists.

But due to a time anomaly, they're "sped up by a million times" and have come back a little too soon--they see their original selves testing the time window! Invisible to their "frozen" past-selves, everyone escapes through the time-window to another dimension or something as the movie then repeats on endless loop for no good reason.