Michael Chambers is trapped in a prison-like cell where a voice asks him if he's ready to eat and asks him to conserve water. Chambers lights a cigarette and wonders how he got into his current predicament. he thinks of how it was a normal day on Earth when creatures from outer space, the Kanamits, arrived on Earth. One craft lands near the UN and a 9' tall representative of the Kanamits enters. He speaks with a mechanical voice and declares that his race's intents are honorable. They want to help the people of Earth and establish embassies. Upon request the Kanamit explains that they can prevent natural catastrophes and offer whatever it is various nations which to accept. He offers to show them a new atomic power source and denies having any ulterior motive. He also offers an impenetrable force field that any nation can use to protect itself. The Kanamit sets down a book and leaves.
Chambers, a decoding specialist for the U.S. government, is called in to decipher the book that the Kanamit left behind. UN translator. He has no success but believes the Kanamits have so far proven themselves trustworthy. The military representatives are skeptical but Chambers notes that they could soon be out of a job if the Kanamits' promises prove out. His assistant Penny comes in and reveals they've translated the title: "To Serve Man."
The UN gathers to review a tape of the government interrogating the Kanamit using a lie detector. They ask him if what he said earlier is true and he confirms that the Kanamits seek to eliminate suffering, war, and hunger. Impressed, the nations of the Earth begin to disarm and offer thanks to the Kanamits for bring on a new era of peace. As peace spreads across the Earth, the Kanamits open up an exchange problem and start transporting people to Earth. Everyone believes that the Kanamit planet is a paradise with everything anyone could ever want. Humans starts leaving while Chambers find himself all but out of a job without military encryption to deal with. Penny notes that the planet seems oddly changed now that there's peace, and there are thousands of Kanamits on Earth. Thousands of Earthlings have departed and Chambers is surprised at how easily everyone has adapted to the knowledge of life on other planets. Both of them are on a waiting list for the exchange program to Kanamit. Penny admits she's still working on translating the Kanamit book.
Eventually Chambers' flight is scheduled and he goes to the departure port. As he prepares to leave, Penny arrives and yells out to him not to get onto the ship. She calls out that "To Serve Man" is a cookbook. The Kanamits force Chambers onto their ship and depart. In the present, Chambers initially refuses to eat, but the Kanamits insists that he eats so he doesn't lose weight. A desperate Chambers finally gives in and starts eating.