The planet where an energy being lures the Enterprise and a Klingon crew in Day of the Dove, beginning a fight it feeds on.
Beta XII-A is the planet that Kirk's landing party visits in Day of the Dove, answering a distress call from a Federation colony. They find no colony at all, and a damaged Klingon ship commanded by Kang arrives, its captain accusing the Federation of attacking his vessel.
Both crews have been deceived by an alien entity that feeds on hatred. It brings both groups aboard the Enterprise, traps them and stokes their conflict, until Kirk and Kang realize that only by laughing together can they drive it away.