Triplanetary describes a galaxy-wide war between two cosmic powers, the benevolent Arisians and the malevolent Eddorians. While the Eddorians use physical agents to corrupt races towards darker ends, the Arisians make more surreptitious plans to help humanity defy the machinations of the Eddorians. The first two "books" describe events in mankind's pre-spaceflight years, while Book III (taken from the 1934 serial) chronicles an extra-solar invasion leading to an opening of the conflict between the Arisians and Eddorians.
Book I: Dawn
1. Arisia and Eddore: The background and history of the generous Arisians (god-like humanoids who work to develop the mind) and the Eddorians (shape-shifting aliens from another space-time, hungry for conquest) are described. When the Arisians learn of the Eddorians' arrival in their galaxy, they mentally cloak their existence from the invaders and make plans for a future generation of Guardians who will defeat the Eddorians for them.
2. The Fall of Atlantis
- b. Arisia: The Arisian Elder Thinkers make plans to cultivate members of a Galactic Patrol who will be able to uses "Lenses" against the Eddorians. They also continue to hide their existence from the Eddorians, for fear of their enemy preparing a defense against them.
- c. Atlantis: On Earth, ruled by Atlantis, an atomic war is brewing between competing nations. An Atlantean agent named Phryges enters Norheim and rams an enemy doomsday missile with his own ship. However, several other missiles do manage to explode, causing the sinking of the continent of Atlantis.
3. The Fall of Rome
- a. Eddore: Gharlane meets with the Eddorian All-Highest to discuss his recent efforts at destabilizing the development of democracy on Tellus (Earth). He has been posing as Nero in Rome. The opposition they face (due to the undetected actions of the Arisians) are assumed to be coming from internal struggles within the Eddorian council.
- b. Arisia: The Arisian Watchman Eukonidor is informed that in order to maintain their hidden hand, they must regretfully allow Rome to fall.
- c. Rome: Gladiators form a plot to assassinate Nero, but at the critical moment Nero/Gharlane uses his alien mental powers to immobilize his attackers, who are subsequently executed. Rome falls.
Book II: The World War
5. 1941: At the beginning of WW II, Ralph Kinnison takes a job at an ordinance manufacturing plant, where he gets fired for his honesty.
6: 19--?: Despite Ted Kinnison's efforts as a defensive technician/pilot, an atomic war breaks out on Earth. Fortunately, the democratic nation of the United States manages to survive the aftermath. Mankind eventually develops spaceflight capabilities and colonizes the Solar System. The Arisians decide to take a more active hand in their struggle against the Eddorians.
Book III: Triplanetary
7. Pirates of Space: Two agents (Costigan and Bradley) from a Triplanetary space liner are taken hostage by space pirates lead by a man named Roger (the Eddorian Gharlane in his latest incarnation). A female passenger named Clio is also captured.
8. In Roger's Planetoid: Costigan, Bradley and Clio use their knowledge of ultrawave technology (advanced telescopy) to escape from Roger's robot-manned base. Unfortunately, while en route back to the Triplanetary Fleet, they and their pursuers (Roger's forces) are soon enveloped by a mysterious "red veil".
9. Fleet Against Planetoid: The Triplanetary Fleet forms cone-shaped battle formations, from which they attack the robot-manned defenders of Gray Roger's artificial planetoid base. However, in the heat of the struggle, a massive ship from the Nevia system arrives, throwing everything into red-tinged disarray.
10. Within the Red Veil: The amphibian Nevians launch an expedition from their star in search of the rare, much-sought after element iron. When they arrive in the distant Sol system, the Nevian captain Nerado captures Costigan and his two friends and destroy the Triplanetary Fleet with their advanced weaponry (a red-tinged energy field which dissolves iron into liquid form for collection and storage). They then destroy Roger's planetoid base, but Roger is able to use an Eddorian weapon to effect a desperate escape. On the Nevian ship, Costigan is able to send a transmission to the Triplanetary League describing their mysterious captors.
11. Nevian Strife: Costigan and his friends are taken to the Nevians' home planet, and are ordered to report to a laboratory for study. However, underwater war-fortresses manned by fish-like enemies of the the Nevians attack, and in the confusion the humans escape the Nevian spaceship in a lifeboat.
12. Worm, Submarine and Freedom: While completing repairs on their lifeboat, the humans are attacked by a giant worm which draws them underwater. After escaping the worm, the lifeboat is next attacked by the Nevian fishmen. However, the amphibian Nevians arrive and during the ensuing battle Costigan manages to lift his lifeboat off the planet.
13. The Hill: The Triplanetary communications scientist Lyman Cleveland investigates the wreckage left over from the battle with the Nevians and returns to the Hill, the home base of the Triplanetary Service. There, he meets with the genius engineer Brad Rodebush and the Triplanetary head Virgil Samms, as the Service works to complete modifications to their greatest warship, the Boise, a vessel armed with an inertialess drive and capable of travelling vast distances in an instant.
14. The Super-Ship Is Launched: When Rodebush and Cleveland launch in the Boise, the ship vanishes without a trace and is assumed to have been destroyed. In the meantime, Triplanetary scientists begin reverse-engineering some of the iron-based science of the Nevians for their own use. In short order, another Nevian ship attacks Earth itself and Terran forces are sent to Pittsburgh to try and halt the iron-extracting rays of the Nevian starship. However, the Terran ships are nonetheless overmatched.15. Specimens: Costigan and his friends are recaptured by Nerado and brought back to Nevia for examination. Eventually, the Nevians place the three humans in isolated observation globes (as museum exhibits for the general public).
16. Super-Ship In Action: The Boise returns from her test flight and engages the Nevian ship over Pittsburgh. With its greater firepower, it drives the Nevian ship back into space and goes in pursuit. Unfortunately, after an accidental explosion the Boise is forced to land on a planet to make repairs.
17. Roger Carries On: After escaping the destruction of his planetoid by the Nevians, Roger and his generals find a new planetoid on which to develop a new home base. During this time he uncovers the existence of the Arisians, and realizes that the Eddorians have been under their "spell" of forgetfulness for millennia. Later, the Boise soon appears in the area and after a heated battle Roger's forces are completely wiped out. Although Roger's physical form is destroyed, Gharlane mentally journeys back to Eddore to warn the Eddorian High Council of the Arisian opposition.
18. The Specimens Escape: Costigan tricks the Nevians into allowing him access to laboratory materials, after which he promptly formulates poison bombs to kill his jailers. After commandeering a swift, armed speedster, he manages to also liberate Bradley and Clio, after which they head back towards Earth. Days later, they notice that Nerado and his warship are in pursuit. Worse, the Nevian ship returning from Earth is also on course to intercept them from the opposite direction.
19. Giants Meet: The Boise picks up Costigan's mayday calls and picks up the Earth trio. Afterwards, it intercepts the Nevian ship escaping from Earth and destroys it. After following Nerado's warship back to Nevia, the Boise then destroys a major city on the Nevian homeworld. Realizing that further conflict would only result in a massive waste of life on both sides, the Nevian and Triplanetary forces decide to form a peace treaty.