"Let That Be Your Last Battlefield" is the fifteenth episode of the third season of the original American science fiction television show, Star Trek. It was first broadcast January 10, 1969, on NBC and repeated August 12, 1969. It was written by Oliver Crawford, based on a story by Gene L. Coon (writing under his pen name "Lee Cronin") and directed by Jud Taylor. The script evolved from an outline by Barry Trivers for a possible first season episode called "A Portrait in Black and White". The script was accepted for the third season following budget cuts. The episode guest-stars Lou Antonio and Frank Gorshin, best known for his role as The Riddler in the Batman live-action television series.

In the episode, the Enterprise picks up the last two survivors of a war-torn planet, who are still committed to destroying each other aboard the ship.

On stardate 5730.2, the Federation starship Enterprise is on a mission to help decontaminate the polluted atmosphere of the planet Ariannus, when sensors track a Federation shuttlecraft reported stolen from Starbase 4. The craft is disabled and brought aboard along with its strange alien pilot, who is found injured and taken to sick bay. The man later awakens and identifies himself as Lokai, a political refugee from the planet Cheron, who requests asylum. Lokai's most striking feature is that his skin is half black and half white, the two halves split perfectly down the center of his body. Science Officer Spock remarks that his physiology may be "one of a kind".

Shortly thereafter, sensors detect another spacecraft in fast pursuit of the Enterprise. Curiously, the craft remains invisible to all but sensors and sets itself on a direct collision course with the ship. Moments later, Spock reports that the invisible craft has disintegrated and deposited an "alien presence" aboard the ship. Captain Kirk turns to see the alien pilot who has beamed himself directly to the bridge. The second alien identifies himself as Bele. Like Lokai, Bele is half black and half white, with the color divided by a line through the exact center of his face. However, the sides of Bele's black and white skin are reversed from those of Lokai, a difference which seems inconsequential to the Enterprise crew, but of great importance to Bele, Lokai, and, apparently, their civilization.

...
Wikipedia