The episode begins with Michael Chambers locked alone in a spartan room with a cot. A voice offers him a meal, delivered through a small aperture in the wall, which he grimly refuses.
The setting changes to several months earlier, on Earth. The Kanamits, a race of 9-foot-tall (2.7 m) aliens, land on Earth as the planet is beset by international crises. As the Secretary-General announces the landing of aliens on Earth to the worldwide public at a United Nations news conference, one of them arrives and addresses the assembled delegates and journalists via telepathy. He announces that his race's motive in coming to Earth is to provide humanitarian aid by sharing their advanced technology, including an atomic generator that can provide electric power for a few dollars, a nitrate fertilizer that can end famine, and a force field that can be deployed to prevent international warfare. After answering questions, the Kanamit departs without comment and leaves a book in the Kanamit language, which leads to Michael Chambers, a US government cryptographer, being pressed into service.
Initially wary of an alien race who came "quite uninvited", international leaders begin to be persuaded of the Kanamits' benevolence when their advanced technology puts an end to hunger, energy shortages, and the arms race. Trust in the Kanamits seems to be justified when Patty, a member of the cryptography staff led by Chambers, decodes the title of the Kanamit book: To Serve Man. The Kanamits submit to interrogation and polygraph, at the request of the UN delegates. When declaring their benevolent intentions, the polygraph indicates that the Kanamit is speaking the truth.
Soon, humans are volunteering for trips to the Kanamits' home planet, which they describe as a paradise. Kanamits now have embassies in every major city on Earth. With the U.S. Armed Forces having been disbanded and world peace having been achieved, the code-breaking staff has no real work to do, but Patty is still trying to work out the meaning of the text of To Serve Man.
The day arrives for Chambers's excursion to the Kanamits' planet. Just as he mounts the spaceship's boarding stairs, Patty runs toward him in great agitation. While being held back by a Kanamit guard, Patty cries: "Mr. Chambers, don't get on that ship! The rest of the book To Serve Man, it's... it's a cookbook!" Chambers tries to run back down the stairs, but a Kanamit blocks him, the stairs retract, and the ship lifts off.
Chambers is in the shipboard room now, and is again offered a meal. He throws it to the floor, but a Kanamit retrieves it and encourages him to eat, to keep Chambers from "losing weight". At last Chambers says to the audience: "How about you? You still on Earth, or on the ship with me? Really doesn't make very much difference, because sooner or later, all of us will be on the menu... all of us." The episode closes as Chambers gives in and breaks his hunger strike.