The Framing Device of the novel is the first-person Decoy Protagonist recounting the death of a famous physicist named Paul Dupont, who has died along with his wife in laboratory explosion eleven years prior. Paul's will named the protagonist as their son Jean's custodian, giving the protagonist access to all their notes. A few years before that, the protagonist witnesses a strange event involving a purple Negative Space Wedgie striking the hydroelectric power plant where Paul (just an engineer at the time) worked. When Paul wakes up, he behaves strangely and even speaks in an unknown language for a bit. Paul Dupont becomes a world-renounced physicist, publishing groundbreaking papers that overturn the human understanding of the nature of the universe. Later, Paul explains that he is actually two people: his friend Paul Dupont and Haurk Akéran, the Supreme Coordinator in the time of the Great Twilight. Paul/Haurk claims that he is from the distant future. A few years later, the protagonist witnesses a similar Negative Space Wedgie killing Paul and his wife. The protagonist finds a package that Paul sent him before his death, detailing Haurk's story. The rest of the novel is that story.
In the story, Haurk explains that modern human civilization is approaching its end. He isn't sure how civilization falls, but before that happens, humans will explore Mars and Venus. What follows are three ice ages with relatively short breaks between them, lasting about 200,000 years. After that, humanity starts from roughly the Paleolithic levels of development, and it takes another 10 millennia to go from this state to first settlements and true civilization. Haurk is born in the year 4575 of the new era. The new humans are roughly the same but have slight neural changes that, basically, means that they don't have any stupid people (by our standards, at least). Thanks to their smarter general population and free flow of information, scientific progress runs ridiculously fast (e.g. they land on Mars and Venus only a few decades after discovering atomic power). Venus is eventually terraformed and settled. Then Earth (which the future humans call Helléra) is invaded by Starfish Aliens called Drums who crush human defenses in a matter of weeks and enslave humanity for over 1000 years. La Résistance manages to create a Synthetic Plague tailored to Drum physiology, and the surviving conquerors depart, destroying as much as they can. They do, however, leave behind the concept of "space magnets", which allow rapid sublight travel between large celestial bodies. Civilization on Earth is rebuilt with the help of the colonists on Venus (whom the Drums didn't bother conquering).
Hyperspace is discovered, but out of 16 hyperspace-capable ships sent to other stars, only one returns revealing that navigation is pretty much impossible in hyperspace, making every trip a Blind Jump. After that, interstellar travel is abandoned, especially since conventional travel using "space magnets" results in a spacecraft being stopped by a natural barrier halfway between any two stars.
Haurk explains that he becomes the head of the observatory on Mercury studying sunspots. One day, he discovers that the Sun is going to explode in 10 years. Less than a year later, he is made Supreme Coordinator of a newly-created organization whose goal is to construct gigantic "space magnets" on the poles of Earth and Venus in order to move the planets beyond Pluto's orbit to wait out the solar explosion, using the resources of all of humanity. Then another scientist named Kelbic reveals that not only will the solar explosion reach much farther than Pluto, but the Sun will become a black dwarf after that. As such, the only option is to take the two Planet Spaceships to the nearest star Etanor (presumably Proxima Centauri). The mass of the planets (plus the Moon) is enough to punch through the interstellar barrier.
Earth and Venus are slowly shifted from their orbits and begin to head to Etanor. Six months before the expected deadline, Haurk discovers that the Sun will explode in two weeks. An archaeologist named Klobor discovers an alien spacecraft under the surface of Mars and chooses to stay behind, studying the craft's engines and transmitting results until the planet is consumed by the solar fire. The two planets manage to escape and continue on their journey. This journey, taking 15 years, is called the Great Twilight. After finally reaching Etanor, the humans find out that the system is inhabited. A human Lost Colony exists on one of the planets (called Tilia) in the system, who are at war with Human Aliens called the Triis from the neighboring planet Kaleb. The Tilians refuse their Earth and Venus cousins permission to stay, claiming that there is always a chance that the far numerous Earthlings and Venusians may one day try to eliminate the Tilians. A militant culture, the Tilians can't afford to take this chance. They recommend that the planets continue to a star called Belul two light years away (there is no such star in Real Life, the closest candidate would be Luhman 16 at 3.6-ly which wasn't discovered until 2009). In exchange for weapons and literary texts, the Tilians agree to send some of their warriors to accompany Earth, as they have previously received messages from Belul indicating a hostile Lost Colony.
In the meantime, Kelbic figures out the function of a strange circuit aboard the reconstructed Martian ship. It's a Time Travel device, allowing ancient Martians to safely explore other stars using hyperspace by using the circuit to jump to a time period when the barrier wasn't there. As the planets cross the barrier between Etanor and Belul, Earth is attacked by a number of fusion Space Mines. Kelbic takes three ships and flies ahead to scout out the Belul system. They are attacked, and two of the ships are destroyed. Kelbic is captured by the local humans and a red-skinned three-eyed Human Alien. The local humans claim that Earth exiled their ancestors and assume that Earthlings have arrived to conquer Belul. Kelbic is taken to a fortress, which is then attacked by a large Earth fleet. Opposed by local humans and the three-eyed Telbirians, the troops manage to rescue Kelbic. They find out that the Telbirians have Psychic Powers that allow them to place humans under More than Mind Control. The controlled humans think that they're in charge and forget all about their true masters if captured. Using a memory-retrieval device on one of the captives, Haurk is able to find out that the Telbirians (actually called R’hneh’er) were a primitive tribe on Telbir until the human colonists began to use them as servant. Learning from humans, the R’hneh’er eventually rose up and took over. They use humans to develop technology and eat those who are too weak or stupid. Kelbic manages to weaponize the memory-retrieval device to work at a distance. After a number of Space Battles, the device is used to get the enemy fleet to perform a Heel–Face Turn (any R’hneh’er aboard are usually killed by their former slaves in the first few seconds). After the device is used on Telbir, the Telbirian humans massacre the R’hneh’er. Earthlings manage to save only about 20,000 and relocate them to an outer planet. The grateful Telbirian humans ask to be allowed to join the Earth-Venus union, and the two spaceship planets are slowly placed into stable orbits around Belul.
The story ends with Haurk explaining that his experiments with the temporal circuit is what caused him to be sent back in time. In his notes, he resolves to try to recreate the experiment to send his consciousness forward into his own body.
The novel provides examples of:
- Adam and Eve Plot: The two Lost Colonies discovered in the novel are started by less than two dozen people, the crews of hyperspace-capable ships sent out to explore the stars.
- Alien Invasion: Not long after future humans begin to once again explore the Solar System and even terraform Venus, Earth is invaded by Drums, who crush all military resistance in a matter of weeks an impose a harsh rule for over 1000 years.
- Anti Matter: An anti-matter disaster on a Venusian powerplant results in the planet being covered by a cloud for over a millennium. While this is obviously bad for the colonists, it spares them the fate of their Earth cousins, who are conquered by the Drums. Thanks to the thick cloud cover, the Drums don't even realize there is a human colony on the neighboring planet.
- Apocalypse Cult: The main danger to the Planet Spaceship project (besides the Sun, of course) is a group of religious fanatics calling themselves Destinists, who believe that it's the destiny of humanity to be destroyed by the Sun's explosion. In their quest to keep humans from leaving the Solar System, they first employ a group called the Economists to lobby on their behalf to the Grand Council. When that fails, they form mobs to take over the tekn research centers in many cities. Most are brutally put down with the use of Tulik waves. One city, which has been completely taken over by the Destinists, who then slaughter the majority of the population (both tekns and trills), is leveled with Orbital Bombardment.
- Later, as Earth and Venus are trying to Outrun the Fireball, a Grand Council member argues that the speed of the planets needs to be increased despite the risk to the planetary crust from the extra acceleration. It turns out that he's a Destinist impostor using Latex Perfection who is still trying to destroy Earth and Venus. And it would have worked too, if it wasn't for that meddling Kelbic.
- Apocalypse How: Several cases ranging from Planetary/Societal Collapse to Stellar/Physical Annihilation:
- According to Haurk, our civilization will come to an end at some point after we land on Mars and Venus through an ice age, which will be followed in quick succession (relatively speaking) by two more. Only after the third one will what's left of humanity start over.
- A 1000-year alien occupation leaves Earth in shambles, especially during the alien exodus, when they destroy everything they have built. Earth only recovers because of the Venusian colonists, who escape the notice of the invaders.
- Shortly before the Drum invasion a chain reaction of accidents destroyed the majority of Anti Matter power plants of the Venus terraforming project. The planet was covered with radioactive fog for centuries. Humans hid underground and transplanted Earth life mutated drastically — semi-sentient elephants with two trunks and poison-spitting armoured crocodiles are mentioned. Thanks to the radiation Drums never dared to land on Venus and never discovered Venusian humans. This helped rebuild Earth after the invasion.
- The Sun explodes in roughly 200,000 years, destroying all planets in range and leaving behind a lifeless black dwarf.
- Artistic License – Space: When approaching the Belul system, Kelbic takes a three-ship squadron and flies ahead to scout out the system. The thing is, the planets are moving at a high percentage of the speed of light, even though they have already started decelerating. This means that the ships would also be moving at about those speeds when taking off ahead of the planets. Since the faster you go the move force you need to keep accelerating, those ships should not have been able to achieve speeds sufficient to be able to scout far ahead of the planets.
- Casual Interplanetary Travel: Travel between planets in the Solar System is fairly easy with ships equipped with "space magnets".
- Death Ray: Tulik waves can instantly kill a large group of people in an area by destroying their neurons.
- Exotic Extended Marriage: Due to a mutation, the ratio of females to males in Tilian society is 7:1. As such, polygyny is a necessity to allow the population to grow. Being a typical warrior culture, only males become soldiers (the novel being written in 1960 and all).
- Polygamy is also practiced on Earth, but only by a small percentage of the population.
- Fantastic Caste System: The Earth society of the future is divided into two classes. The tekns (possibly derived from "technician") are the higher-status minority of high-IQ professions such as scientists, doctors, engineers, and some categories of writers. The trills are everybody else. Each class has its own government with the Grand Council being the supreme governing body for both classes. Membership in each class is not hereditary but based on career choices and aptitude test scores when one is a teenager. It's possible for a trill to become a tekn later in life, if he or she shows aptitude in a tekn area. When a person is being officially named a tekn, he or she must swear before the Council of Masters never to divulge unapproved scientific knowledge to trills. The penalty for such an act is a life imprisonment on Pluto. The tekn/trill divide is not as pronounced on Venus as it is on Earth. However, even on Earth, tekns and trills are allowed to freely socialize and marry. Haurk (a tekn) is the son of a trill father and a tekn mother.
- Some Tilian males are born completely lacking initiative. They are useless as soldiers and are kept as servants. This leads to friction between the two.
- Framing Device: Most of the novel is a story left by Haurk to the Decoy Protagonist after he attempts to recreate the experiment.
- Heroic Sacrifice: An archaeologist named Klobor sneaks off the ship carrying the rest of his team to Earth in order to stay behind and study the recently-discovered Martian starship. He knows that he only has days before the Sun explodes but gambles that the alien drive system will allow humanity to explore the stars. He is aided by engineers guiding him in real-time via Hek waves. On the day of the explosion, as the temperature in his lab is rapidly rising, he toasts Haurk with a poison-filled glass and drinks it, collapsing immediately. Haurk and the others keep watching the video out of respect until the camera is finally destroyed.
- Human Aliens: Explorations of the underground city on Mars reveal statues of long-extinct Martians, who used to look very similar to humans. The R'hneh'er are a cross between this (albeit with red/purple skin and three eyes) and Lizard Folk.
- Imported Alien Phlebotinum: "Space magnets" are brought to Earth by the conquering Drums. After the Drums leave, humans begin to use them for Casual Interplanetary Travel.
- Improbable Age: Haurk effectively becomes the temporary dictator of humanity before he's even 30. After the 15-year Great Twilight ends, he is glad to go back to his old job as a scientist.
- In the Future, Humans Will Be One Race: One of the side effects of future humans being descended from the survivors on one large island is that all the races will have merged by the time civilization rises again.
- Lost Colony: Two are discovered in the story, descended from ships lost during the initial attempts to use hyperspace. One is friendly to Earthlings. The other one is not (thanks to More than Mind Control by the R’hneh’er).
- Mega City: Most cities on Earth in Haurk's time are of this type (172 of them, to be exact). The largest one is called Huri-Holdé, located where Casablanca is now, and stretches for 1000 meters (0.62 miles) into the sky, almost qualifying for Star Scraper status and 450 meters (0.28 miles) underground. From the sky, Huri-Holdé appears as a rough circle 75 kilometers (46.6 miles) in diameter. The city has a population of 90 million. The population of Earth at the time is 5 billion. Unlike a typical setting where mega cities are built to protect humans from the harsh Crapsack World, the world outside the cities is a lush paradise where animals roam free, largely undisturbed by humans.
- Mental Time Travel: Haurk's body doesn't travel back in time. Only his consciousness, which merges with the mind of Paul Dupont. This appears to be a malfunction of the temporal circuit, which is normally supposed to send physical objects through time. After the attempt by Haurk to recreate the experiment, Paul Dupont and his wife Anna are found dead. It's not clear if the experiment succeeds in sending Haurk back into his own body or if Paul and Anna go with him.
- Named After Their Planet: Averted with two of the four alien races in the novel. The Triis are from the planet Kaleb in the Etanor system, and the R'hneh'er are from Telbir in the Belul system. However, Kaleb and Telbir could be human names for those planets, so this trope may be true in those races' native languages. The name of the Drum homeworld is never revealed, and very little known about the extinct Martians and what they called themselves (presumably, not Martians, since they probably didn't worship the Roman god Mars).
- Naming Your Colony World: Haurk gives all planet names in French, since they would otherwise be unrecognizable to the Decoy Protagonist reading his notes. He mentions that his people call Earth "Helléra". However, the names of other stars are in his language, such as Etanor (implied to be Proxima Centauri) and Belul (unknown Real Life name, as there is no known star 2 light years from Proxima Centauri). Additionally, the humans in those systems call their planet Tilia (in Etanor) and Telbir (in Belul).
- Negative Space Wedgie: The "purple lightning" that causes Haurk's consciousness to be placed in Paul Dupont's body.
- The "space magnetic" barrier between two nearby stars that prevents physical bodies less massive than a planetoid from crossing. The only way to pass it is with a sufficiently massive object moving at a high percentage of the speed of light.
- Outrun the Fireball: As Earth and Venus are accelerating out of the Solar System, the Sun explodes several months ahead of schedule. A member of the Grand Council wants the planets to accelerate faster in order to escape the coming blast wave despite the risk to the crust from the acceleration. It turns out that he is a Destinist impostor, who tries a last-ditch effort to destroy humanity. Kelbic then explains that there's no need to increase their speed because, when the weakened solar wave catches up to them, the first thing that will happen is the frozen atmosphere will turn to gas again, acting as a shield.
- Planet Spaceship: After it's discovered that the Sun is going to explode, the future humans immediately begin to plan a way to survive. They decide to build giant "space magnet" drives on Earth's poles in order to move the planet to another system. The plan gets amended when a high-ranking tekn points out the prohibitive cost in resources and time to try to build the Northern magnet without any landmass. Instead, they settle for 10 smaller magnets along the Arctic Circle. A portion of the Antarctic is melted to allow the construction of the Southern magnet. Enormous underground shelters are built to allow the survival of humanity as well as many of the animal species (not all are saved, though, due to the limited space). Air and water are conserved in vast underground caverns. Instead of trying to relocate the Venusian colonists to Earth, they decide to move Venus as well. Since Venus lacks polar ice caps, the construction of "space magnets" is easier there (only 2 are required). Since rapidly moving a planet from its intended orbit is likely to shatter the crust, they have to slowly increase the planets' orbital speed to allow them to reach solar escape velocity. Since the Moon is gravitationally tied to Earth, it remains the planet's companion for the entire journey.
- According to the novel's physics, only an object whose mass is at least that of a small planetoid and is moving at the speeds exceeding 80% of the speed of light can pierce the magnetic barrier between two nearby stars. The author seems to ignore the danger of interstellar dust particles at those speeds, even if there is no one on the surface of the planet.
- Player-Guided Missile: The primary weapons of Earth's Space Navy.
- Proud Warrior Race Guy: The Tilians are forced to be soldiers in order to fight off the Triss attacks. As one of them keeps repeating, as a true soldier, he hates war but knows it's sometimes necessary. While they are friendly to their Earth and Venus cousins, they warn Haurk that, should Earthlings and Venusians attempt to attack them, they will have quite a fight on their hands, since the Tilians are constantly at war with the neighboring Triss, and Solar System humans have never fought a Space Battle before. As part of an agreement with the Tilians, a number of their warriors are sent with Earth and Venus to aid them against the Telbirians.
- Ray Gun: Fulgurators are the standard handheld weapons used by humans in Haurk's time. Since "fulguration" is the name for a Real Life medical procedure, it's implied that fulgurators are Improvised Weapons.
- The R'hneh'er use Mind Control to manipulate other civilizations and eat humans. Unusually,they were a stone age society before encountering humans to manipulate.
- Space Mines: Earth is hit by a number of homing fusion mines, left there by the Telbirian humans to intercept any potential Tilian ships. Given that Earth is moving at a high percentage of the speed of light at the time, it's a wonder there isn't more damage.
- Space Navy: Earth builds one in case there are hostiles in Etanor. The Tilians have one as well, although less advanced. However, the Tilians have experience with space combat, while Earthlings don't. Additionally, the Triss (the Tilians' enemies) have more primitive spacecraft (powered by atomic engines instead of "space magnets") but greater numbers. The Telbirians build a large fleet to fight off a possible invasion by the Tilians. It is more primitive that Earth's fleet, .
- Subspace Ansible: Hek waves are used to communicate between planets in real-time.
- Subspace or Hyperspace: The future humans discover hyperspace and attempt to use it, only for all but one ship to be lost due to a Negative Space Wedgie between two nearby stars that throws off hyperspace navigation, resulting in Blind Jumps.
- Starfish Aliens: The Drums are a Descriptively-Named Species (only coincidentally, since this is their name for themselves, and the novel is written in French) with barrel-like torsos, eight legs and seven tentacles.
- Synthetic Plague: La Résistance develops a virus designed to only kill Drums. The Drums assume it's a natural plague. They finally leave after 99.9% of them are dead.
- Terraforming: The future humans terraform Venus to Earth-like standards.
- Time Travel: It's revealed that ancient Martians knew how to travel through time. They used this method to safely navigate the galaxy using hyperspace. Basically, they'd jump to the barrier between the stars, time travel to the time before or after the barrier was there (due to stellar drift), cross the area using "space magnets", time travel back, resume journey using hyperspace. On the other hand, they have explored the galaxy in various time periods, which is evidenced by archaeologists finding photos of Tilia from orbit showing a city that was only built 300 years before Earth's journey, while the Martians themselves have died out half a million years ago.
- To Serve Man: One of the ways the R'hneh'er use their Mind Controlled humans is as food animals.
- Venus: Haurk mentions that, when their civilization first reach Venus, they find traces of humans previously having been there, implying that, at some point in the near future, we will at least visit the hellish planet. In Haurk's time, Venus is a terraformed paradise settled at least 1500 years before the Haurk is born. One of the Anti Matter power plants on Venus explodes, and covers the planet in a radioactive cloud. This is what keeps the Venusian colonists from being detected and subjugated by the invading Drums. The Venusians manage to repair the damage and, after the Drums are gone following a 1000-year reign, help Earthlings rebuild. When the Solar System is threatened by the Sun exploding, it's decided that Venus will also be turned into a Planet Spaceship (slightly faster due to it being only about 80% of Earth's mass), as relocating a planetful of people and building additional underground bunkers for all of them would require greater effort than building two giant "space magnets" on the planet's poles.
- The War of Earthly Aggression: The Telbirian human believe this is the reason why Earthlings and Venusians have arrived to their system. It turns out that all Telbirians are mind-controlled by the R'hneh'er to believe that.
- We Will Have Perfect Health in the Future: Future humans receive an inoculation that protects against any infection, even from an alien pathogen.
- Zee Rust: Given that the novel was written in 1960, this is justified. However, there isn't as much here as one would expect.
