A pig-like alien race called the Kanamit come to Earth, apparently meaning us nothing but good. They provide devices that create free energy, a soil treatment that improves crop production, and--a device that prevents all explosions: "There will be no more war." They're even working on medical treatments to make men taller, stronger, and healthier.
The Ukranian ambassador to the UN, Gregori, doesn't trust them, and the protagonist, a translator, is caught up in his quest to understand their motives. They get their hands on a Kanamit book, "To Serve Man", and try to learn the language.
At last, Gregori announces that he has figured out their motive. He has read the first paragraph of the book: it is a cookbook.