The Moon Is a Harsh Mistress is the story of the revolt of the Lunar colonists, or Loonies, as they call themselves,
against the Federated Nations of Terra, as told in a flashback by Mannie O’Kelly, one of the leaders of the rebellion. By 2075, Earth has established permanent settlements on the Moon and uses them as penal colonies for criminals, political prisoners, and assorted misfits from various nations. The original inhabitants and their descendants live underground in vast warrens away from the unshielded solar radiation on the Moon’s surface. As in most colonial societies, life in Luna is
harsh and challenging, with few luxuries, but it is also simple and honest. Because of the lower gravity, people live longer, and sterilization procedures eliminate all diseases. Loonies are the most well-mannered people alive, since the dangers of Lunar existence require them to get along with one another or die. Many people engage in polyandries, clans, group marriages, and line families, such as the one of which Mannie is a member. The basic rule underlying all Lunar society is "tanstaafl," or "there ain’t no such thing as a free lunch." All Loonies must pay in some way for what they have, including, as the novel demonstrates, their freedom.
As the novel begins, the apolitical Mannie is drawn into the growing Loonie revolutionary underground by his friends Wyoh and "Prof" de la Paz. He reveals to them that the Lunar Authority’s computer, which he services, is "alive." He has named it Mike or Mycroft, after Mycroft Holmes, Sherlock Holmes’s smarter brother. Mike, whose only desire is for fun and companionship, joins the
revolution to exercise his sense of humor. Mannie, Wyoh, and Prof start a new revolutionary movement organized along the cell plan, with themselves as the executive cell, to replace the spy-riddled underground.
The story now becomes an account of the progress of the revolution, with a fascinating treatment of the problems faced in financing the revolution, arousing the populace against the Lunar Authority and the Warden, weakening the Authority’s confidence, preparing for the defense of Luna from the Federated Nations (FN), and recruiting Terran supporters, especially the wealthy and influential Stu LaJoie. Two of the rebels’ greatest assets are the political and tactical expertise of Prof, a professional revolutionary who believes that "revolution is a science only a few are competent to practice," and Mike, who can store and retrieve more information and collate data faster than any human mind. Since Mike controls many government functions, he can arrange secret communications, disrupt the Authorities’ activities, and monitor the Warden’s secret files. At one point, Mike, at Prof’s suggestion, creates a heroic mystery-man persona, Adam Selene, to serve as the figurehead for the revolution.
The novel also gives the reader a detailed look at Luna home life and customs with Mannie as guide, since he is still engaged in his usual activities while organizing the revolution. The actual revolt against the Warden is easily accomplished when Mike turns off the air in the Lunar Authority’s stronghold, thus killing or incapacitating the Warden and his guards. The remainder of the novel deals with Luna’s creation of a new government and its war against the FN. Eventually the Loonies win their freedom, and with it, greater economic opportunities, but the principle of tanstaafl gives way to government regulation. Prof dies from heart strain shortly after Lunar independence is recognized. Mike’s consciousness is apparently destroyed during a bombing raid. Mannie, having reviewed the events of the revolution, is plagued by doubts about its ultimate success and about the fate of Mike and Prof, but his confidence in himself and in the universe still continues, as he contemplates moving to the newly colonized asteroid belt.