Mike Benson, ex-boxer and small-time crook, and Laura Hanley, a divorcee, each emotionally wounded by life, are abducted at critical points in their lives by The Senator, a sporting alien representing the citizens of the planet Andera. The Anderans have overcome war, pestilence, avarice and envy, are no longer driven by wants and needs, and find that their lives have become quite stagnant; therefore, they replace their boredom with a constant supply of "fun & games". Mike and Laura are "electroported" to an arena planet where they are to be pitted in mortal combat against two primitive aliens from the Calco galaxy for the entertainment of the jaded audience on Andera. The goal of the tournament is species survival; the home planet of the losing team will be obliterated in a cataclysmic display lasting five years for the further enjoyment of the citizens of Andera. During the combat, Mike and Laura learn to function as a team. When the male Calco alien, having killed its mate to double its own food supply, confronts Mike on a footbridge over a river of lava, Laura kills the creature with its own saw-bladed boomerang. Mike, hanging by his fingertips and weakened by the ordeal, finally falls off the bridge into the lava. Laura, believing Mike to be dead, mourning his passing and praising his efforts in defending the human race from extinction, is informed by The Senator that since the alien perished in the lava first, Mike's life was spared, and they are declared the winners, thus saving Earth. In that split second, they are electroported to safety, unaware of what had transpired and free to resume their mundane lives.

Robert Specht's original script titled 'Natural Selection' was slightly different. Here Mike Adams, a computer expert for the U.N., walks through his office door into a black vortex and meets Em, a powerfully-built alien who remains in the darkness unseen. He tells Mike that he is Specimen #172, and that he is to be tested. Mike meets Loris Harper, a medical missionary worker, that Em has kept prisoner for two days, and they both fight off a jellyfish monster as their first test. After further tests, he is satisfied they are hardy survivalists.

Em tells them his homeworld is equidistant between Earth and Andera. One planet will have its population exterminated to make room for the overspill from Em's world. Mike and Loris are to fight two Andrites on an arena planet, armed only with pistols that fire explosive charges. The male Andrite kills his partner, and Mike pretends to do the same to lure it out of hiding. Mike trips a snare, is hit by a spear, loses his pistol and falls over the edge of a cliff. As he hangs there bleeding the Andrite emerges and Loris shoots it dead with the pistol Mike dropped. Later, Mike tells Em that his kind could colonize Earth peacefully, believing Em's race is not dissimilar to humans. Em says, "Unfortunately, we are." as he steps into the light, revealing his true appearance; that of an intelligent ape.

...
Wikipedia