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The Skylark of Space
Richard Seaton, a chemist, semi-accidentally discovers a metal that can catalyse the total conversion of matter to energy, and with help from his wealthy friend Martin Crane, uses it to build a spaceship, the "Skylark". His unscrupulous rival Marc Duquesne steals enough information and samples to build a second ship, and plans to force Seaton to step aside so Duquesne and his associates in World Steel corporation can control the discovery. Duquesne kidnaps Seaton's fiance, Dorothy Vaneman, as leverage, but an accident sends Duquesne's ship into deep space, with Dorothy and another hostage, Margaret Spencer, on board. Seaton and Crane pursue and rescue them, but are forced to use most of their copper fuel reserves, and lose track of their course; they put Duquesne on parole until the situation is resolved.
The party lands on the inhabited planet of Osnome, seeking more copper, and finds the population to be technologically advanced but ignorant of atomic energy. They help the nation of Kondal to win a war of extinction with its rival Mardonale, and the Kondalians help to rebuild and refuel the Skylark as "Skylark Two". Richard and Dorothy are married while on the planet, as are Martin and Margaret. They then navigate back to Earth, where Duquesne escapes.
Skylark Three
Dunark of Osnome visits Earth, in a ship based on the Skylark, and requests help in fighting off an invasion from neighbouring planet Urvania, which also lacks atomic power but otherwise has far superior technology to Osnome. However, while the Skylark is flying to assist, both ships are attacked and nearly overwhelmed by a scout ship of the highly advanced Fenachrone. Seaton narrowly defeats and destroys it using an experimental weapon, but learns that the Fenachrone will destroy Earth in retaliation, and are planning to conquer the entire galaxy.
Using captured Fenachrone technology, Seaton forcibly ends the war between Osnome and Urvania, and searches for technology that can help to beat the Fenachrone; Osnomian and Urvanian legends about visitors from the stars lead him to find the ancient planet Norlamin, home to a race of Actual Pacifist scientists. Although unwilling to directly kill, the Norlaminians build the party a large new ship, "Skylark Three", based on their "fourth order" and "fifth order" technology, and help to issue an ultimatum to the Fenachrone, demanding that they pull back their forces. When the Fenachrone refuse, Dunark destroys their entire homeworld using a fifth-order weapon, and the party pursues and destroys the few escapees using Skylark Three.
Skylark of Valeron
In deep space after killing the Fenachrone, the Skylark encounters a group of disembodied intelligences, which attack in an attempt to learn more about Seaton's experiments in the "sixth order". To escape, the party abandons Skylark Three and launches Skylark Two through four-dimensional "hyperspace", arriving in a far distant galaxy with no reference points.
Duquesne, meanwhile, tracks down Norlamin and tricks the inhabitants into helping him, claiming to be an ally of Seaton. He then uses their technology to subdue Earth's military forces and declare himself its overlord, before fortifying the planet with fourth- and fifth-order equipment. The Norlaminians are dismayed to learn of the deception, but unprepared for war, and are thus unable to break his control.
In a far-off galaxy, the Skylarkers rescue a nearby planet, Valeron, from invading "Chlorans" — hostile chlorine-breathing amoebas — and use the planet as a base of operations to build a planetoid-sized craft, the "Skylark of Valeron". With Seaton's discoveries about the sixth order, they are able to locate and return to Earth, which they liberate from Duquesne's control. After capturing the "pure intellectuals" that they previously encountered, Seaton turns Duquesne into another disembodied intelligence, and sends them all away in a prison ship intended to carry them out of the universe.
Skylark Duquesne
Several years after Skylark of Valeron, Seaton and Crane have introduced Earth to cheap atomic power and interstellar flight. This attracts attention from the Jelmi, a group of refugees from the oppressive Llurdi in a distant galaxy, who have discovered a controllable means of travel through the fourth dimension. Duquesne, escaped from the prison ship after an accident and restored to flesh, pursues and contacts the Jelmi under a false name, and is able to obtain the technical details of the fourth dimensional device. However, when he attempts to use it to send assassins onto the Skylark, it draws attention from a nearby galaxy full of Chlorans, who launch a massive attack that destroys most of the Skylark and scares Duquesne off.
After their narrow escape, Seaton and party covertly visit a planet within the Chloran galaxy, seeking more information, and help an enslaved nation to avoid annihilation, giving them a base of operations, but they are still unable to overcome the Chlorans' overwhelming numbers advantage. Meanwhile, Duquesne builds a ship, the "DQ", comparable to the Skylark of Valeron, crews it with a previously unknown group of Fenachrone survivors, whom he rescues from the Llurdi, and attacks the Chlorans by launching bombs through hyperspace. However, in the process, he discovers that there are hundreds of millions of Chloran worlds, too many to effectively fight, and they are expanding. He retreats and contacts Seaton for a truce to fight the greater threat.
The Norlaminians then reveal that some of the people whom the Skylark saved from the Chlorans are psychics, and that Seaton and Duquesne also have latent gifts. They combine their psychic abilities with fourth dimensional transition to manipulate stars, allowing them to destroy the entire Chloran galaxy at extreme range; the Chlorans attempt to fight back with their own psychics, but the Fenachrone assist Duquesne in fending off their attack until the extermination is complete.
In the aftermath, Duquesne leaves to establish a new and superior race with himself as emperor, marrying Stephanie de Marigny, who approves of his plans.
