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.
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.
[1] Joseph Stefano rewrote 'Natural Selection' as there were too many costly aliens and visual effects called for, and that the testing procedures endured by Mike were too complicated, redundant or riddled with technical gibberish.
[2] The Calco Alien mask (without its bulging eyes) and the taloned hands, made by Projects Unlimited, were reused (with an added hairy bodysuit) in the Star Trek pilot "The Cage".
Thanks to the plot, which keeps jumping back to the murder of the Poker Dealer, Mike's frantic search for a place to hide in Laura's apartment building is seen three times, and many of the shots on the Arena planet are seen twice.