TOS Season 1 Episode 26 Errand of Mercy

Plot

Negotiations between the United Federation of Planets and the Klingon Empire have collapsed, and the USS Enterprise is sent to the world of Organia, a non-aligned planet near the Klingon border, to prevent the Klingons from taking advantage of its strategic location. As the Enterprise approaches Organia, the ship is attacked and destroys a Klingon vessel.
Upon arriving on the planet's surface, Kirk and Spock find a peaceful but technologically primitive town. Kirk appeals to Ayelborne, the head of the local council to allow the Federation to help them resist Klingon occupation, but the Organians adamantly oppose any use of violence. When a Klingon fleet appears in orbit, Kirk orders the Enterprise to withdraw, which strands himself and Spock on the planet.
Ayelborne disguises Kirk and Spock to avoid suspicion and the Klingons seize control of the planet without resistance, installing the ranking Klingon officer, Kor, as the military governor. Kor has Spock questioned with a "mind sifter" device to confirm he is not a spy and designates Kirk as the Organian civilian liaison with the occupation force. Spock uses his mental discipline to withstand the scrutiny of the device.
That evening, in an attempt to inspire the Organians, Spock and Kirk sabotage a munitions dump outside the town. Ayelborne reveals the true identities of the Federation officers to Kor. He and the council then mysteriously free the two before the Klingons can torture information out of them. While Kirk and Spock try to comprehend the council's contradictory actions, Kor orders the execution of two hundred Organians, yet the council seems unmoved.
As the Federation and Klingon fleets ready themselves for a confrontation in the system, Kirk and Spock raid the Klingon headquarters in hopes of rousing the population into resistance. They capture Kor and prepare to make a last stand. The Organians then reveal their true nature: they are highly advanced incorporeal beings. They instantly incapacitate both sides, forcing them to agree to a cessation of hostilities. The Organians predict that the two sides will work together in the future.