The story is set in what appears to be the present time (i.e., 1950), in cold war America, and is told in first-person narrative by a United Nations translator. The story opens at a special session of the UN where three alien emissaries, the pig-like "Kanamit," are testifying that the purpose of their mission to Earth is "to bring to you the peace and plenty which we ourselves enjoy, and which we have in the past brought to other races throughout the galaxy." The aliens soon supply Earth with cheap unlimited power, a device that suppresses explosions, and drugs for prolonging life. As a further token of friendship, they allow humans to visit their home planet via ten-year "exchange groups."
A friend of the narrator, a UN translator named Gregori, steals one of the Kanamit books, and he and the narrator attempt to translate it, via a basic Kanamit-English dictionary provided by the aliens. After some weeks, they determine that the title is "To Serve Man." Two weeks later, the narrator returns from a trip to find Gregori distraught. Gregori says that he has translated the first paragraph of the book:
"It's a cookbook," he said.