Relations between the United Federation of Planets and the Klingon Empire have reached boiling point, and recent negotiations have broken down. The Starship Enterprise has been sent to Organia, a non-aligned planet near the border, in fear that the Klingons will annex it.
Upon approach the Enterprise is attacked by and destroys a Klingon vessel. It is soon discovered that war has officially started. Captain James T. Kirk and his landing party warn the Organians of the Klingons, but they are not concerned. The Klingon fleet then invades, forcing withdrawal of Enterprise leaving Kirk on the surface.
The Klingons invade, and Kirk and Spock start guerilla operations against the occupation force. They are discovered, and in retalation, the Klingons start mass execution of Organians. Federation and Klingon fleets converge on the system, threatening to turn it into a war-zone.
At that point, the Organians reveal their cards, and disclose that they are not humanoids with primitive technology as they may appear, but are highly advanced beyond even the Klingons and the Federation. They force the opposing sides to make a truce, which would later be known as the Organian Peace Treaty.