The Enterprise investigates a planet in the vicinity of another starship's disappearance, but the crew is warned of danger as they approach the planet. Detecting no actual threat, Kirk and Spock beam down, where they are told that the Enterprise crew has been labeled a casualty of a recent attack—a simulated attack in a simulated war with a neighboring planet. Now Kirk and his crew—like generations of people before in this war—are expected to willingly walk into "disintegration chambers" where their deaths can be tallied.
This episode is a good example of an anti-war message the way only TOS could tell it. It drops the subject under a spotlight of absurdity, and has Kirk take a defiant attitude in an effort to change these people's backward ways. Ultimately, he changes their minds by giving them no choice: Either they give up the simulated war and declare peace, or they fight war with real weapons and real destruction, destroying the society they've tried to preserve. It's strangely amusing how Kirk's bold-and-brash in-your-face attitude can make a story work.