In the year 3000, Earth has been ruled by an alien race, the Psychlos, for a millennium. The Psychlos discovered a deep space probe (suggested to be Voyager 1) with directions and pictures mounted on it and the precious material, gold, that led them straight to Earth.

After one thousand years, humanity is an endangered species numbering fewer than 35,000 and reduced to a few tribes in isolated parts of the world while the Psychlos strip the planet of its mineral wealth. Jonnie Goodboy Tyler, a young man in one such tribe, lives in the shadow of the Rocky Mountains. Depressed by the recent death of his father and both the lethargy and sickness of most of the surviving adults in his tribe, later determined to be caused by radiation-leakage from decaying nuclear land-mines, he leaves his village to explore the lowlands and to disprove the superstitions long held by his people of monsters in those areas. He is soon captured in the ruins of Denver by Terl, the Psychlo chief of planetary security.

Psychlos stand up to 9 feet (2.7 m) tall and weigh up to 1,000 pounds (450 kg). They originate from Psychlo, a planet with an atmosphere radically different from Earth, located in another universe with a different set of elements. Their “breathe-gas” explodes on contact with even trace amounts of radioactive material, such as uranium. The Psychlos have been the dominant species across multiple universes for at least 100,000 years. It becomes apparent in the later chapters that the Psychlos were originally non-violent miners but were subjugated by a ruling class called “Catrists” to become malicious, sadistic sociopaths.

Terl has been assigned to Earth, and his term has been arbitrarily extended by Numph, the planetary head of mining operations. Fearful at the thought of spending several more years on Earth, Terl decides to make himself a multi-millionaire to escape, by secretly mining a lode of gold in the Rocky Mountains that his planetary scanner drones have recently found. It is surrounded by uranium deposits that make Psychlo mining impossible, so Terl decided to capture a Man-Animal to mine the gold for him.

Terl forces Jonnie to submit to a learning machine programmed by a servile race that was exterminated centuries earlier for going on strike. It quickly teaches him numerous subjects, including the Psychlo language, by implanting the information directly into Jonnie’s brain. He befriends a Psychlo midget named Ker, who is only 7 feet tall but still possesses the impressive strength of a Psychlo, and is markedly less psychotic than the others.

Looking for leverage against Jonnie, Terl captures his childhood-love Chrissie and her sister, Pattie, who went searching for Jonnie a year after he left their clan, and holds them hostage to ensure his continued cooperation. Thereafter, Jonnie is free to move around the mining area. Terl and Jonnie travel to Scotland where Jonnie recruits eighty-three Scottish people to help with the mining, including several deliberately selected body-doubles for Jonnie, older women to perform the cooking and clothes mending, a doctor, a teacher, and a historian. Jonnie tells the Scots about the evil deeds of Terl, to include how he has imprisoned Jonnie’s love and her little sister. Led by Robert the Fox, the Scots agree to help him fight against the Psychlo rule on Earth and rescue Chrissie and Pattie. Terl does not understand English, and is instead convinced that the Scots are motivated by a promise of pay on project completion.

While Jonnie and his Scottish allies mine the gold deposit, they also secretly explore the ruins of humanity to look for uranium that can be weaponized for use against their Psychlo oppressors. This subterfuge is aided by the aforementioned body-doubles, making it appear to Terl’s surveillance that the mining operation is the sole priority of the human contingent. Meanwhile, Terl finally gains leverage on Numph, discovering that he has been stealing company funds. Terl blackmails him, effectively negating Numph’s power over him, allowing Terl to continue with his mining plans.

Terl has been busy obfuscating the purpose of the gold-mining operation and implementing his plan to ship the human-mined gold back to the Psychlo home-planet. Terl’s plan involves replacing lead coffin-lids with lead-plated facsimiles made from the gold mined by the Scots, and shipping these coffins with dead Psychlos in them, home. When he finally returned to Psychlo, he could then dig up the coffins and sell the lids to make his fortune. All dead Psychlos are to be returned to home planet for burial, but recent safety measures have reduced accidents. Terl thus has to manufacture accidents to kill Psychlos, and decides to assassinate Numph as well, to get the bodies needed.

During the semi-annual teleportation of personnel, goods, and coffins to Psychlo, Jonnie and his allies co-opt Terl’s plan by packing the coffins with “dirty nukes” and “planet busters” they have found, and replacing the golden coffin-lids with the original lead lids. After the last teleportation, the humans use the Psychlos’ own weapons against them and gain control of the planet. With humans in control of Earth, Jonnie works to discover the secret of Psychlo mathematics and teleportation. This is a difficult task, compounded by the fact that Psychlo math is based on the number eleven, and Psychlo equations appear to make no sense.

Before the teleportation, Jonnie is forced to oppose a longtime rival from his own clan, Brown Limper Staffor, who is seeking to wrest control of Earth for himself. Unwittingly used by Terl to advance his own plans, Brown Limper nearly succeeds after gaining assistance from a group of cannibalistic mercenaries from southern Africa called the Brigantes, and their leader, General Snith. But Brown Limper is killed by Terl just before the Psychlo’s teleportation, and the Brigantes are defeated.

It is discovered that all Psychlos have a deep brain-stimulation device implanted in their brains to make them controllable. Meant to make work pleasant for them, the device promotes extreme sadism in the males, causing them to attack any non-Psychlo who shows interest in Psychlo mathematics and teleportation. If the Psychlos are unsuccessful in killing their intended victims, the device compels them to commit suicide. The removal of this device frees the handful of remaining Psychlos on Earth from its affects. Curiously, Ker did not have any such device implanted in his brain.

With the Earth being threatened by other alien races looking for restitution because they had suffered under the harsh rule of the Psychlos, Jonnie opposes a race of intergalactic bankers seeking to repossess the Earth for unpaid debts. The security and independence of humanity once again threatened, Jonnie redoubles his efforts to figure out Psychlo teleportation.

It is eventually discovered that the dirty nukes sent with the intent of destroying the capital city on Psychlo instead started a chain reaction which reached into the planet’s core due to over-mining, causing the planet to explode and transform into a star. Jonnie also discovers that other Psychlo facilities scattered about the multiple universes were destroyed by their own reliance on teleportation as they performed their scheduled teleportation shipments, and instead, brought back radioactive solar matter. This holocaust killed every single Psychlo in the multiple universes except for the handful remaining on Earth. Once it is revealed that all female Psychlos who leave the homeworld are sterilized to prevent off-world births, Johnny realizes that the Psychlos on Earth will not be able to reproduce, and eventually, the Psychlo race will become extinct.

Jonnie then works out a way to prevent the repossession of Earth via contracts Terl had signed with Brown Limper Staffor. The Psychlo had thought that it would be amusing to make Staffor believe that he was the legal owner of Earth as well as all Psychlo possessions across the multiple universes, by signing a contract that stated as much before his final teleportation to Planet Psychlo. Terl had no way of knowing that he was about to die, along with almost his entire race, with the destruction of his homeworld. Once planet Psychlo was destroyed, Terl was the highest ranking member of the Intergalactic Mining Company left alive, and his signature on Staffor’s contract became legal. That meant that Jonnie, as the recognized leader of Earth with the death of Brown Limper, now owned what was left of the entire Psychlo empire. Using these contracts, the Earth Planetary Bank pays off all debts to the intergalactic bankers.

However, Jonnie is still perplexed by Psychlo mathematics. With the help of an aged Psychlo engineer, he learns about Psychlos using a cipher system and dummy equations to make their mathematics unsolvable. At the same time, he also discovers how the Psychlos protected their teleportation technology in their local equipment, and records the circuits for future use. Using the existing teleportation console, Jonnie is able to bring back breathe-gas from a planet in the Psychlo star system that was never officially recorded. With the Psychlo math and the circuits, Earth begins to manufacture teleportation equipment, sold to numerous planetary systems via the intergalactic bankers. At the same time, Jonnie uses the Earth’s newly acquired wealth to buy impenetrable force fields and automated orbiting defense platforms to protect the Earth from future threats.

With the Earth secure and the human population growing and learning about its true history, Jonnie gives ownership of the Earth back to its people. A few years later, Jonnie and Chrissie are married and they have a son and a daughter. With human civilization being rebuilt and thriving, Jonnie and Chrissie take their children and leave for an isolated part of the world to train them in the old ways of survival, and to live out the rest of their lives in peace. But, after a year, their friends find them and implore them to return to civilization, which Jonnie reluctantly agrees to.

Years later, frustrated with un-ending fame and life away from nature, a middle-aged Jonnie takes some supplies and quietly slips away to the Rocky Mountains, never to be seen again. He becomes a figure of legend.