The Invincible is ostensibly a traditional science-fiction adventure novel whose tight and engaging plot describes an investigatory expedition to a distant planet. It is, in addition, a parabolic commentary upon the difficulties of human and nonhuman communication.

The narrative begins with the landing of the spaceship Invincible on the planet Regis III in the outermost quadrant of the Lyre Constellation. The goal of the crew, led by Horpach, the spaceship commander, with the assistance of Rohan, the navigator, from whose perspective subsequent events are recounted, is to retrieve a lost spaceship, the Condor. Satellite photographs reveal a series of geometric formations suspected to be abandoned cities. These prove instead to be lifeless, impenetrable, metallic mounds of wiry tangles supported within by huge pillars which intersect innumerable rods and folded layers of honeycomb structures.

The crew also locates the Condor. Its interior is in shocking disarray, and dead crewmen are scattered in and about the ship. It appears that the Condor crew died of natural causes, although one Invincible scientist suggests mass insanity. Rohan leads a second squad to the abandoned “cities” but is ordered to return to the Invincible when a member of another investigative group returns to the craft a helpless amnesiac. A prospecting expedition under the command of Regnar sets off to analyze minerals but instead comes in contact with “furiously dancing...sparkling black iron crystals” that form an immense cloud and swallow up the expedition’s scout. Dispatched to rescue this party, Rohan finds most of Regnar’s men in a confused stupor. Four of the team’s members—Regnar, Benningsen, Korotko, and Mead—have disappeared.

Lauda, a physicist, proposes that intelligent life from the Lyre Constellation may have brought robots and computers to Regis III in an unsuccessful effort to escape an exploding supernova. While the voyagers died, their robots adapted and flourished. A process of machine evolution took place, however, in which solar energy and miniaturization enabled the predecessors of the “tiny pseudo insects” to triumph. The ruins of the “cities” found earlier are the remains of the inanimate self-reproducing structures that lost the battle for existence to the “insects.”

Rohan and his party return to the Invincible, having been attacked by a swarming black cloud which killed one man and left the remainder infantile. From samples captured by Rohan, it is learned that the “insects” are Y-shaped with tiny wings and crystalline structures. When joined together, these “insects” can fly, generate heat, and produce electrical or magnetic fields to protect themselves.

Horpach sends a huge robot ship, the Cyclops, to track down the four missing men. The “insects” attack it. In defending itself, the Cyclops produces an enormous and devastating conflagration. The Cyclops emerges outwardly unscathed, but the Invincible crewmen quickly realize that it has lost its bearings and is wandering aimlessly around the planet. Subsequently, the defeated Cyclops attacks the Invincible and is destroyed by a tremendous shot of energy from the mother ship. Following the attack, Horpach, who now appears quite human and frail, admits that he is unsure whether to leave or to remain on the planet and asks Rohan to make the decision. Rohan resents this abdication of responsibility but grudgingly agrees to search for the missing men in what will be a very dangerous solitary mission.

In the course of the one-man reconnaissance, the cybernetic “insects” attack Rohan, but by remaining immobile he experiences no ill effects. He stumbles upon the bodies of three of the four men, witnesses strange behavior by the “insects,” and becomes convinced that the expedition should leave Regis III. Rohan finds an abandoned vehicle which carries him safely to the Invincible.