Defiant

The fourth and final book in the Skyward series opens with Spensa, a young human starfighter pilot, floating in the nowhere, a void outside of space and time. During the previous book's events, Spensa merged her soul with Chet, a delver, one of the eldritch beings that exist beyond normal reality and can destroy entire planets. This fusion has made her a weapon the delvers fear, though she does not yet know how to use her new abilities. She searches for M-Bot, her former AI starfighter who survived as a bodiless consciousness after his physical form was destroyed, but cannot find him. Using her enhanced cytonic powers, a form of psychic ability, she glimpses memories left by her friends during her absence: Jorgen Weight, her boyfriend and fellow pilot, reluctantly assuming military and governmental command after the destruction of Detritus's political leadership in a bomb trap set by Winzik, the Superiority's dictator; and new alliances with the kitsen, small foxlike aliens, and the UrDail, violet-skinned humanoids. Spensa returns to her body on Detritus, the shielded planet that serves as humanity's home.

At Jorgen's promotion to admiral and provisional head of government, Spensa learns the full scope of their predicament: Winzik's production capacity will soon let him field thousands of fighters and hundreds of capital ships. During the briefing, Spensa suffers an uncontrollable cytonic episode in which objects teleport randomly and her fears broadcast into everyone's minds, revealing how unstable her merged state with Chet has become. M-Bot then contacts her telepathically, having survived by hiding among the delvers.

Spensa proposes cutting off the Superiority's supply of acclivity stone, the material that powers all starships. Her childhood friend Rig, now chief engineer, confirms that without it the enemy's ships cannot fly. Cuna, the sole senior Superiority defector on their side, reveals that only five mining stations exist, all operating through portals between the somewhere and the nowhere. The coalition agrees to raid the Superiority's secret information nexus to locate these depots. The nexus turns out to be hidden on Luna, Old Earth's moon, in the Sol system where humanity originated.

The raid proves morally devastating. The nexus is a full civilian city, not a military outpost. When Nedd, one of Spensa's flightmates, is severely wounded and loses an arm, Spensa's unstable powers erupt: she rips apart buildings and uses debris as weapons, destroying the city's defenses and setting much of it ablaze. A ground team steals the entire data archive, but Spensa is shaken by the destruction she caused. That night, Brade, Winzik's human cytonic operative, appears to Spensa as a psychic projection, praising her destructive power and attempting to recruit her. Spensa rejects her.

The data reveals five mining operations and the locations of twelve other human preserves. Jorgen plans simultaneous strikes on all five supply depots, but the flight debates the morality of attacking installations full of civilian workers. Kimmalyn, Spensa's closest friend in the flight, states plainly that the Luna raid was wrong. Spensa investigates a portal beneath Detritus and discovers it has been booby-trapped by the delvers to capture cytonics, explaining the disappearance of the kitsen's psychic adepts and of Gran-Gran, Spensa's grandmother.

Rather than destroy the depots and kill civilians, Spensa devises a secret plan. Gran-Gran, the last living crewmember of the original starship that brought humanity to Detritus, intercepts Spensa before she can leave alone and insists she take friends. With Kimmalyn, her flightleader Arturo, and the UrDail pilot Alanik, Spensa hyperjumps to one depot, bluffs the local officials, rescues captive taynix (intelligent cytonic slugs that serve as living hyperdrives), and hires Peg and the Broadsiders, her pirate allies from the nowhere, to lock all five mining portals. The mission succeeds without loss of life, but Jorgen is furious that Spensa's unilateral action undermined his authority and damaged trust with their allies.

Cut off from acclivity stone, the Superiority plans an all-out assault. Brade challenges Spensa to a starfighter duel; Jorgen refuses to authorize it. Spensa spends time building a new ship body for M-Bot alongside Hesho, the former kitsen emperor who serves as her copilot and bodyguard. But while working alone, she suffers another cytonic episode. Brade appears physically, exploiting the way episodes weaken Detritus's protective inhibitor field, a cytonic-suppression barrier. She stuns Spensa, captures Doomslug, Spensa's pet taynix, and hyperjumps them to enemy territory.

Spensa is brought before Winzik at the Brez Observation Platform near Evensong, the Superiority's central communications hub. Winzik displays her to the delvers as proof of a deal: the Superiority will silence all cytonics in exchange for the delvers' cooperation. Drugged to suppress her abilities and imprisoned, Spensa attempts multiple escapes, all thwarted. A captive commslug she names Comfort reveals that thousands of enslaved taynix support one another through a secret emotional network. When Comfort tries to contact Jorgen, Winzik detects the signal and executes the slug.

Brade stages a fake escape and a mock duel to extract information about Spensa's delver bond. Afterward, she shoots Winzik dead, revealing she has been the true power behind the Superiority's military, with Winzik as her puppet. She explains her motivation: raised as a weapon, she needs to fulfill the destiny of conquest to prove her lifetime of abuse had purpose. She tricks Spensa into delivering a killing blow to Winzik's dying form on camera for propaganda.

Detritus hyperjumps into the Evensong system after M-Bot contacts Jorgen with news of Spensa's capture. Gran-Gran commands the newly built flagship Defiant, a battlecarrier named after the original ship. Coalition forces try to disable the inhibitor stations surrounding Evensong to let Detritus jump closer, but the captive slugs refuse to leave their posts, unwilling to abandon the other taynix trapped on the hub. Gran-Gran devises a desperate gambit: she sacrifices the Defiant as bait, evacuating her crew and remaining alone on the bridge. The enemy fleet converges and destroys the ship, but coalition fighters convince the inhibitor slugs behind the Defiant to let Detritus's gun platforms hyperjump through. The platforms obliterate the concentrated enemy fleet. Gran-Gran dies a hero's death.

Spensa's drug wears off, and her soul ejects from her body as a ghostlike projection. M-Bot stalls the delvers by impersonating one and planting doubt about Brade through their constant mental blending, a process of checking one another for uniformity. Spensa steals an antidote from Brade, restores her powers, and fights Brade in a ferocious brawl. Brade orders the execution of all thirty thousand taynix on Evensong, but her own security lockdown prevents compliance. She flees toward the station in a starfighter.

Spensa enters the nowhere and confronts the delvers. M-Bot reveals that as a being of both worlds, she can force the passage of time into the timeless nowhere, stripping away the delvers' protection over their buried grief. Rather than freeze them in eternal agony, Spensa chooses compassion. She separates from Chet and shows the delvers he has healed by living with pain rather than hiding from it. The enslaved taynix, despite their own suffering, reach out to comfort the delvers, who begin to heal. When Brade orders the taynix killed, the enraged delvers send drones to rescue every captive slug across the galaxy, depositing them on Detritus.

Hesho arrives on a modified missile, fights through Brade's rear guard, and frees Spensa. M-Bot forms a new ship body by manifesting in the somewhere, just as delvers create physical forms. Spensa climbs in with Doomslug and Hesho, contacts Skyward Flight for a roll call, and pursues Brade. Surrounded by her entire flight, Spensa vaporizes Brade's cockpit with a single shot.

In the aftermath, the taynix are granted citizenship in the Defiant coalition and will decide when each species earns back interstellar travel. A new galactic alliance forms to replace the Superiority, and the delvers and taynix negotiate a treaty limiting hyperjumping. Chet creates a human body and begins learning to live as a person. Spensa mourns Gran-Gran but finds peace knowing she died on her own terms. Jorgen commissions Spensa to lead an exploration flight mapping dangerous, unexplored planets and searching for the mystery of Earth's disappearance. Kimmalyn, Chet, Hesho, Doomslug, and M-Bot will join her. Spensa looks up through a gap in Detritus's platforms toward the stars, feeling she belongs out there but recognizing that the lights of her friends shine brighter.