The episode opens with a man simply known as Chambers. Reclining on a bed, a voice on a speaker asks what he would like to eat, to which Chambers replies he does not wish to eat at that time.
In a voice-over, Chambers narrates how on an April day, flying saucers began to appear all over Earth. Inside the saucers were alien beings called Kanamits, nine foot tall creatures who communicate telepathically but use a 'mechanical voice' to converse with humans. One addresses the United Nations offering friendship and asserting that they intend to help mankind solve all their pressing global problems including hunger and energy shortages.All they ask in return is to be trusted. Before leaving, the ambassador accidentally leaves a book in the UN chambers.
It is then revealed that Chambers was a decoding specialist for the military, and was tasked with translating the Kanamits' book. After some time, Chambers and his associates manage to translate the title, which reads: "To Serve Man."
The Kanamit ambassador agrees to answer questions under a lie detector, as many still doubt the Kanamits' intentions. During the session, the ambassador explains that their methods of eliminating hunger, war and famine have been used to the benefit of other civilizations throughout the galaxy, and that the elimination of these threats to the human race would be sufficient reward for their services. The lie detector shows no traces of falsehood in the ambassador's testimony, so the results are broadcast to the world.
All the nations of the world eagerly voice their support for and trust in the Kanamits. As promised, their technology ends hunger and solves the world's energy crisis and other ills, and the nations of earth eventually lay down their arms in peace. Subsequently, the Kanamits begin ferrying humans to their home planet in their saucers, as thousands respond to the reports of paradise-like conditions there. One criteria for boarding is that each human must be weighed before entering a saucer.
Meanwhile, as the earth has demilitarized, Chambers and his assistant find themselves with very little to decode. As Chambers prepares to take a trip to the Kanamit's home planet, his assistant concentrates her efforts at deciphering the information contained in the Kanamit book left behind.
As Chambers boards a saucer in New York, his assistant rushes to him but is stopped by one of the Kanamits. While struggling to free herself from the being's grasp, she screams to Chambers that the book "To Serve Man" is actually a cookbook! Chambers struggles to free himself to no avail and is forced aboard by another Kanamit.
The episode ends with Chambers in his room being prodded to eat more. Chambers turns to the camera and asserts that whether you are on Earth or on the ship, it doesn't matter because sooner or later 'you'll be on the menu.' Chambers. consigned to his fate, then sits down to eat the food prepared for him by the Kanamits.