 While on a vital mission to decontaminate the entire planet Ariannus, where a terrible disease rages, the Enterprise suddenly encounters in rapid succession two small vessels, each with only one humanoid occupant which they take on board. The men are from a single planet, but of two genetically most improbable races, left and right halves of the skin being either white viz. black or the other way around. It turns out that Lokai is a refugee, belonging to the slave race which rose in rebellion and is therefore relentlessly chased throughout the galaxy, since many years, by commissioner Bele of the arrogant master race. Even on board their obsessional fight is resumed; Bele abuses his extraordinary mental power to force by his will the ship to deliver Lokai on their planet and thus delay its crucial mission far too long, leaving Kirk no other choice then bluff by starting the self-destruct sequence...