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Triplanetary
The Arisians, ancestors of all life in the universe and masters of telepathic power, encounter the newcomer Eddorians, who have arrived in the universe from elsewhere and intend to conquer it. Recognising that the Eddorians would be able to stalemate their best attacks, the Arisians instead suppress the Eddorians' memories of encountering them, and covertly begin a selective breeding program among their child races, to produce a superior race capable of killing the Eddorians.
After thousands of years of Arisia raising up Earth's civilisations and Eddore knocking them down, humans reach a point where interplanetary spaceflight is possible. Their early ships are attacked by the amphibious Nevians, who need iron to power their technology, but a disguised Arisian, Nils Bergenholm, provides humanity with a device that neutralises inertia, allowing travel vastly beyond light-speed, and they fight the Nevians to a truce and technology exchange, now with interstellar capabilities. However, in the process, one of the Eddorians encounters the Arisians' interference and overcomes the memory block, letting him know about the shadow war.
First Lensman
The Eddorians step up the level of piracy, vice and oppression that their servants are creating, prompting Virgil Samms (an intermediate result of the Arisian breeding program) to seek further help. Nils Bergenholm directs him to visit Arisia, where Samms is given the first Lens, granting him telepathic powers and resisting all attempts to steal or counterfeit it. Samms then seeks to create an interstellar organisation for law enforcement and unified government, the Galactic Patrol, to be led by Lensmen. He is opposed by politicians who belong to the Eddorians' organisation, "Boskone", but ultimately succeeds in winning the North American presidential nomination for his friend Roderick Kinnison (another intermediate Arisian result), arranges for many of the Boskonian ringleaders to be exposed and arrested, and fends off an assault by a Boskonian fleet afterward.
Galactic Patrol
Many years after Virgil Samms, the Galactic Patrol holds sway over a large coalition of planets, with many thousands of Lensmen and lesser officers governing it, but Boskone has recently escalated their ongoing conflict, with new warships that are faster and more powerful than the Patrol's. Kimball Kinnison, a newly recruited Lensman (and penultimate result of the Arisian breeding program), comes to prominence after helping to obtain samples of the cosmic conversion technology that powers the Boskonian ships, but in the process, recognises his own limitations, and returns to Arisia afterward for further training. Mentor of Arisia puts him through "second stage" treatment, expanding and organising his mind and increasing his ability to tracelessly invade and coerce other minds, with or without a Lens. Kinnison then uses this ability to infiltrate and trace the Boskonian organisation to its leader, Helmuth, whose planet the Patrol assaults and destroys. In the process, he becomes acquainted with nurse Clarissa McDougall, Virgil Samms' descendant (and another penultimate result of the breeding program), and they begin to fall for each other.
Grey Lensman
Having spotted signs that Helmuth was actually reporting to a higher authority, Kinnison investigates the nearby galaxy Lundmark's Nebula, finding that Boskone is already there in force, but also gaining access to technology that allows entire planets to be moved around faster than lightspeed. He then goes undercover in a variety of roles, following the drug trade, until he is able to track down the planet of the Eich in the second galaxy, ruled by the Council of Boskone. In the process, he is captured and severely injured, but escapes and is nursed back to health by Clarissa again. The Patrol destroys the world of the Eich by crushing it between mobile planets, and Kinnison and Clarissa become engaged.
Second Stage Lensmen
Mentor interrupts Kinnison to warn him that the council of Boskone was not the end of the threat, and that he needs to continue his efforts against them instead of settling into an office job. He works with the few other second-stage Lensmen to track down the planet of Thrale, ruled by a disguised Eddorian, and Kinnison infiltrates it as an apparently loyal Boskonian officer, rising quickly through the ranks to become the Tyrant. He then leads the Boskonian fleet into an ambush by the Patrol, which has invaded the second galaxy in force, and the Boskonian fleet is defeated, while Kinnison (with concealed help from Mentor of Arisia) overcomes and kills Gharlane of Eddore, who was secretly advising and controlling the previous Tyrant. Once Thrale is subdued, Mentor permits, and even encourages, Kinnison and Clarissa to marry.
Children of the Lens
Approximately two decades after their marriage, the Kinnisons have a son and four daughters, all with extraordinarily powerful minds, and their son has just graduated as a Lensman. Kimball has noticed a resurgence of activity suggesting that Boskone is not entirely gone, and sets out to investigate, with the children pursuing their own avenues, including the four girls each shadowing a second stage Lensman and helping him out without being discovered. Their research converges on the planet Ploor, home to the Eddorians' immediate subordinates, which the Patrol destroys. The Arisians then assemble all the Lensmen in existence for a final assault on Eddore itself, successfully breaching its shields and snuffing out all life inside, before the Arisians retire and leave the universe, leaving the children as guardians of life henceforth.
Masters of the Vortex
Set at around the same time as Second Stage Lensmen, this is an unrelated story set in the Lensman universe. For years, atomic power plants have been exploding spontaneously into slowly expanding vortices of energy that absorb all they touch and pollute the surrounding atmosphere, and nobody can figure out why. After losing his family to an ill-advised attempt to destroy one of them with explosives, physicist and mathematical savant Neal Cloud becomes motivated to wipe them out, but it's dangerous work and he's the only one who can do it.
In an effort to find a mechanical/computerized solution, he's paired up with a cyberneticist named Joan Janowick, who's also a self-taught telepath. When she teaches it to him, it unlocks the full potential of his mind. He's then able to communicate with the Cahuitans, alien entities that the two of them have already deduced must be responsible for the vortices, and to find an amicable solution which will end the menace permanently.
In the course of all this, he becomes involved in a battle between hitherto-undiscovered aliens and with various criminal conspiracies that touch on the edges of the Boskonian war, with some of the events and people he meets along the way leading to the development of his character and abilities before the final denouement.
