William Mandella is a physics student conscripted for an elite task force in the United Nations Exploratory Force being assembled for a war against the Taurans, an alien species discovered when they apparently attacked human colonists' ships. The UNEF ground troops are sent out for reconnaissance and revenge. The elite recruits have IQs of 150 and above, are highly educated, healthy, and fit. Training is grueling – first on Earth and later on a planet called "Charon" beyond Pluto (written before the discovery of the actual planetoid). Several of the recruits die during training due to the extreme environments and the use of live weapons. The new soldiers complete training and immediately depart for action via interconnected "collapsars" that allow ships to cover thousands of light-years in a split second. However, crucially, traveling to and from the collapsars at near-lightspeed has enormous relativistic time effects.
Their first encounter with Taurans, on a planet orbiting Epsilon Aurigae, triggers their post-hypnotic training, which causes them to massacre the Taurans despite their lack of resistance. This first expedition, beginning in 1997, lasts only two years from the soldiers' point of view, but due to time dilation, they return to Earth in 2024.[4] During the expedition's second battle, the soldiers experience future shock first-hand, as the Taurans have much more advanced weaponry. Mandella, with fellow soldier and lover Marygay Potter, returns to civilian life, only to find humanity drastically changed. He and the other discharged soldiers have difficulty fitting into a society that has altered almost beyond their comprehension. The veterans learn that, to curb overpopulation, which led to class wars around the world caused by inequitable rationing, homosexuality has become officially encouraged by many of the world's nations. The world has become a very dangerous place due to mass unemployment and the easy availability of weapons. Alienated, Mandella and many other veterans re-enlist, despite the extremely high casualty rate and their recognition that the military is a soulless construct. Mandella and Potter receive promised postings as instructors on Luna, but upon arrival are immediately reassigned to a combat command.
Almost entirely through luck, Mandella survives four years of military service, while several centuries elapse for humanity in general. He soon becomes the objectively oldest surviving soldier in the war, attaining high rank through seniority rather than ambition. He and Potter (who is his last link with the Earth of his youth) are eventually given different assignments, meaning that even if they both survive the war they will likely never meet again due to time dilation. After briefly contemplating suicide, Mandella assumes the post of commanding officer of a "strike force", commanding soldiers who speak a language largely unrecognizable to him, whose ethnicity is now nearly uniform ('vaguely Polynesian' in appearance) and who are exclusively homosexual. He is disliked by his soldiers and he assumes this is because they had to learn 21st century English to communicate with him and other senior staff and because he is heterosexual.
Engaging in combat thousands of light years away from Earth, Mandella and his soldiers need to resort to medieval weapons to fight inside a stasis field which neutralizes all electromagnetic radiation in anything not covered with a protective coating. Upon return, the strike force learns this is the last battle of the war. Humanity has begun to clone itself, resulting in a new, collective species calling itself simply Man. Man is able to communicate with the Taurans, who are also clones. It is discovered that the war started due to a misunderstanding; the colony ships were lost to accidents and those on Earth with a vested interest in a new war used these disappearances as an excuse to begin the conflict. The futile, meaningless war, which had lasted for more than a thousand years, ends.
Man has established several colonies of old-style, heterosexual humans, just in case the evolutionary change proves to be a mistake. Mandella travels to one of these colonies (named "Middle Finger" in the definitive version of the novel) where he is reunited with Potter, who had been discharged much earlier and had taken trips in space to use time dilation to age at a much slower rate, hoping for Mandella's return. The epilogue is a news item from the year 3143 announcing the birth of a "fine baby boy" to Marygay Potter-Mandella.
