The story opens aboard an interplanetary liner cruising through the ether, suddenly attacked by ruthless space pirates employing outlawed lethal gas. Conway Costigan, a vigilant first officer operating secretly for the Triplanetary League's elite service, thwarts the immediate sabotage but cannot prevent the capture of the liner by an immense artificial planetoid commanded by a mysterious, ruthless mastermind known as Roger. Along with Clio Marsden and Captain Bradley, Costigan is imprisoned in the mechanical fortress. Roger attempts to bend the captives to his will through scientific domination, but the persistent secret agents maintain covert communications via sub-ether instruments.
Escaping their cells after disabling robotic guards, Costigan and Bradley recover their heavy space armor and devastating energy weapons. They smash through the planetoid's primary power room, neutralizing its generation systems just as an even more powerful, fish-shaped spacecraft belonging to the amphibians of Nevia arrives. These advanced alien beings strip the planetoid of its immense iron stores to fuel their own technological expansion, absorbing pirate and Triplanetary forces alike into a vast fluid stream of allotropic iron.
Captured alongside Roger's remaining forces by the Nevians, the three humans are treated as biological specimens and subjected to rigorous laboratory examinations. Costigan, feigning submissiveness, utilizes his laboratory access to synthesize liquid gas, unleashing a devastating strike that incapacitates the inhabitants of the Nevian city. Seizing a newly tested Nevian space-speedster, Costigan rescues Clio and Bradley, initiating a desperate flight toward their home solar system while pursued by Nevian commanders.
Meanwhile, back at the mountain stronghold known as the Hill, scientists Lyman Cleveland and Frederick Rodebush finalize the reconstruction of Triplanetary's ultimate deterrent: the Boise, a super-ship capable of inertialess space travel and powered by intra-atomic iron energy. Launching into the void, the super-ship intercepts Costigan's distress signal, closing the interstellar distance in mere moments.
The narrative culminates in a titanic clash of civilizations as the Boise confronts both the pursuing Nevian cruisers and the remnants of Roger's forces. Utilizing newly developed macro-beams and focused semi-material drills, the super-ship shatters the enemy's impenetrable polycyclic screens and disintegrates their advanced armaments. With the immediate threats neutralized, Costigan and Clio find solace in each other's arms, knowing their survival secures the continued freedom of the Three Planets.
