TOS Season 3 Episode 15 Let That Be Your Last Battlefield

Plot

The Federation starship Enterprise is on a mission to help decontaminate the polluted atmosphere of the planet Ariannus, when sensors detect a Federation shuttlecraft that was reported stolen. The craft is brought aboard along with its alien pilot, who identifies himself as Lokai, a political refugee from the planet Cheron. Lokai's most striking feature is that his skin is ink-black on one side of his body and chalk-white on the other side.
Shortly thereafter, sensors detect another spacecraft in pursuit of the Enterprise. The alien craft disintegrates, but not before its pilot, Bele, transports to the Enterprise bridge. He is colored black and white, similar to Lokai. Bele explains that he is on a mission to retrieve political traitors. His current quarry is Lokai, whom he has been chasing for 50,000 Earth years. Bele is taken to Lokai, and the two begin to argue about the history of their peoples, almost coming to blows.
Bele demands that Captain Kirk take him and Lokai to Cheron. Kirk refuses, telling him he will have to make his case to Federation authorities. Some time later, the ship changes course to Cheron, and Bele announces that his "will" has taken control of the ship. Lokai demands the death of Bele, and Kirk orders both of them to be taken to the brig. Unfortunately, a force field generated by both aliens makes that impossible. With no other way to regain control, Kirk threatens to destroy the Enterprise, and begins the ship’s auto-destruct sequence. In the last seconds of the countdown, Bele relents, and the ship resumes its course to Ariannus.
As Bele continues angrily to press his matter with Starfleet, he reveals the source of his conflict with Lokai. He, and all of his people on Cheron, are black on their right sides, while Lokai's people are all white on their right sides. The distinction is lost on the ship's officers, who leave it for legal authorities at the next starbase to decide, as the Federation has no extradition treaties with Cheron, and the case requires due process. Once the Ariannus mission is completed, Bele takes control of the Enterprise again, this time disabling the self-destruct system. When the ship arrives at Cheron, Spock can find no sign of intelligent life. Lokai and Bele realize they are each the only ones left of their peoples, who have completely annihilated themselves in civil war. Enraged, they attack each other, their force fields threatening to damage the ship. Lokai breaks away, Bele pursues him, and the two eventually beam down to the planet. The bridge crew remark sadly on their unwillingness to give up their hate.