Two beings from the same world who hate each other over their skin colour carry their war aboard the Enterprise.
The Enterprise is hunting for a stolen shuttle when it finds a being, Lokai, who is half black and half white. Soon after, another being, Bele, who is also half and half but with the colours reversed, arrives and claims Lokai is a criminal. They have been fighting for thousands of years, and Bele tries to take over the ship.
The story is a parable about racism, which Kirk sees as absurd, since the difference between them is only which side of their face is black. It is often described as heavy-handed, but it is also one of the most direct commentaries on prejudice in the series.