Michael Chambers, a decoding specialist with the U.S. government, is in a sealed chamber when a voice states it's meal time and ask for his preference. Chambers tells the voice to get lost and refuses to eat. He thinks back to how he got into the situation he's in and how no one on Earth considered the future...
In April, the nations of the world worried about the various problems plaguing the Earth. Flying saucers arrived from space and landed at various spots across the Earth. The UN warns that there is no sign that the aliens, who call themselves Kanamits, have any hostile intent. One of the Kanamits, a 9' tall bald alien, arrives at the UN and declares his race's peaceful intent. They want to open embassies and arrange immigration back and forth between the two planets. The Kanamit explains that they can help Earthlings avoid or prevent natural disasters and provide a near-infinite power source. The Russian ambassadors wonders if the Kanamits have an ulterior motive but the representative asserts that their motives are totally altruistic and they can provide each nation with a force field to protect itself from any enemy attack.
Chambers is called in to decipher a Kanamit book that the alien representative leaves behind, but has no luck. He believes that the Kanamits are what they claim, noting that they've already restored the soil fertility in South America to demonstrate their good will. His assistant Penny deciphers the book's title: "To Serve Man."
The UN representatives meet to see a tape showing an interrogation of the Kanamit using a lie detector. After establishing a baseline for physical measurement, they confirm that his mission is to bring peace and plenty to everyone on Earth. The nations of the Earth begin to disarm and come together in peace. The Kanamits turn deserts into farmlands and armies are all but disbanded as the force fields are distributes. Meanwhile, the Kanamits set up an exchange program and promote how pleasant their home planet is. Chambers, with nothing to decode for military intelligence, shuts down his office as he chats with Penny and talks about how easily Mankind has adapted to the news of alien life. He admits that he is on a exchange group waiting list and Penny notes she is too. She is continuing to work on the Kanamit book but so far hasn't had any success.
Later, Chambers is ready to depart. However, as he is on the landing ramp, Penny arrives to tell him that she's translated the book: it's a cookbook. The Kanamits force Chambers onto the flying saucer and depart.
In the present, Chambers throws his food to the ground but a Kanamit enters and encourages him to eat so he doesn't lose weight. With no other choice, he finally desperately gives in and starts eating.