After barely surviving Resson, Violet Sorrengail awakens in rebel-held Aretia and discovers her brother, Brennan, lives. In the Assembly chamber, leaders debate scarce resources, a missing luminary needed to forge ward-anchoring alloy, and whether the eight marked cadets should return to Basgiath. Suspicion falls on Violet until Xaden Riorson claims responsibility by custom and two dragons vouch for her. Violet learns of venin advances, iron lures of Navarrian make, and a dormant Aretian wardstone. Andarna has abruptly matured into a black adolescent, losing her time-stopping gift, and must enter Dreamless Sleep. Despite risks, the group votes to return to Basgiath to protect marked cadets and keep weapons moving.

Back at the college, Violet and Xaden stage a cover story: a gryphon ambush beyond the wards killed Soleil and Liam and nearly killed Violet. General Lilith Sorrengail accepts the evidence and rebukes Colonel Aetos for abandoning Athebyne. Graduation proceeds; Xaden is commissioned and immediately reassigned to the front. New restrictions follow: Major Varrish is installed as vice commandant, and orders limit Violet and Xaden to fortnightly visits, threatening their dragons’ tolerance. Aetos’s veiled threats trigger Violet’s uncontrolled lightning, and Xaden shields bystanders with shadow.

Orientation unveils a brutal Rider Survival Course with surprise abductions and interrogations. With fewer dragons bonding, the Empyrean hesitates as venin stir. At Conscription Day, Varrish’s dragon, Solas, immolates runners and collateral cadets; Tairn arrives and forces Solas to withdraw, establishing a dangerous feud. Violet retrieves Liam’s letters, shelters grief, and begins covert research with scribe Jesinia into ward origins. A first-year assassin murders Nadine while hunting Violet; Violet kills him and suspects Aetos’s reach.

Violet’s leave to the Samara outpost reveals harsh realities: riders brawl for limited passes, outposts rely on alloy-imbued weapons to bolster wards, and the rebels still lack a luminary. Xaden refuses to barter Violet’s safety for Viscount Tecarus’s terms. Back at Basgiath, Varrish weaponizes Violet’s signet, forcing lightning output until she nears burnout; Tairn plunges her into a river to save her. During a drugged land-nav, bonds and signets are severed to simulate capture. Later, “interrogation training” turns into torture in a hidden facility. Violet’s squad refuses a severing draught, endures beatings, and escapes only after Healer Nolon covertly creates a chance and Violet’s dagger unlocks a warded door.

Leaflets claiming Zolya fell to dragon fire circulate; Markham dismisses them, then unveils Nolon’s secret triumph: Jack Barlowe, presumed dead, has been mended and returns—renewing danger. When Samara is attacked by three drifts, Professor Devera’s signal makes clear Xaden is gravely wounded. Violet defies orders, launches, finds him alive, and they reconcile enough to resume a clandestine dagger drop to Poromiel. At the rendezvous, veteran flier Syrena accepts limited aid while Cat (Catriona Cordella) needles Violet and hints at a luminary. Xaden refuses Tecarus’s bargain and arms Violet with a concealed venin-killing dagger.

With trust strained and surveillance tight, Violet finally confides in Rhiannon, Sawyer, and Ridoc about venin, wardstones, and the daggers. During Threshing, infantry assassins infiltrate and kill Eya; an unseen ally saves Violet from the tower’s brink. A heist into the Archives’ forbidden depths with Jesinia and prince Aaric yields two small journals. Nolon drugs Violet in the courtyard; Varrish seizes her and, with a truth-sayer, tortures her to bait Xaden. Dain Aetos is compelled to read her memories; Violet drags him through Resson, then he turns on Varrish and frees her as an unknown rescuer kills the truth-sayer. Xaden and Garrick cut a path to the cell; Violet executes Varrish. In the courtyard, Tairn projects Resson to the wing, offering a choice. Nearly half the quadrant defects with the marked and two professors to Aretia; Andarna is towed south, still asleep.

In Aretia, chaos reigns as defectors arrive. Training restarts; Andarna wakes changed—able to fly eventually but never to bear a rider. Violet translates Warrick’s journal, learns of an “iron flame,” and attempts to activate Aretia’s wardstone with the blood of six powerful riders; it fails. With the forge still missing a luminary, Violet flies to Cordyn to negotiate despite Xaden’s ban. Tecarus demands a public lightning test; the arena chest breaks open to a caged venin. Violet kills it by pinning its boots with alloy daggers and electrifying floodwater. A deal is struck: weapons and relocating 100 Poromiel fliers to Aretia in exchange for a luminary and intel.

Trust between riders and fliers is forged the hard way on the Medaro Pass, where a gryphon and flier die and wyvern strike from the clouds; Violet kills four with targeted lightning through storm cover. Professor Trissa introduces rune tempering—stored magic that can be triggered later—an equalizer long banned by Navarre. Cat’s emotion-amplifying mindwork nearly provokes Violet into killing her during sanctioned challenges; Xaden intervenes and later reveals protection runes birthed the rebellion relics. Fliers are merged into Iron Squad; Violet recruits a contrite Dain to translate and begin imbuing Aretia’s wardstone. As Pavis falls to venin, Violet corrects a crucial translation: “blood” is “breath,” the “six” are dragons, and activation will end flier channeling. She levels with her mixed squad and offers maorsite-charged conduits as stopgaps.

Sensing misdirection in Melgren’s vision of Samara’s fall, Violet concludes the true target is Basgiath’s Vale. Fourth Wing launches. In the ward chamber beneath the college, Jack appears astride a corrupted Baide and murders her, dousing the wardstone’s black fire with dragon blood; the stone breaks and Navarre’s wards collapse. Jack boasts of planted lures and drains Dain before he’s subdued. A storm summoned by Lilith empowers Violet’s lightning as the horde hits. Gryphons hold ground in rain; riders bleed in the air. Violet rescues Sawyer after a wyvern bites off his leg; on the field she kills a venin, then a stronger one destroys her conduit and immobilizes her—until Andarna erupts from the cliff, reveals new camouflage and fire, and saves her.

Inside, Jesinia reveals Lyra’s journal reads “seven,” not “six,” while a larger wyvern with a royal-blue rider appears. Violet argues to try the ward again; Xaden admits the horde is waiting for him and strides out to buy time. In the chamber, Brennan confirms the stone can hold power. Violet begins to imbue through Tairn and Andarna and nearly burns out. General Sorrengail intervenes, forces Sloane Mairi—the siphon whose power manifested in the caves—to channel Aimsir’s storm through her, and sacrifices herself to light the stone. The iron flame burns; wards rise; wyvern that cross the border die midair.

After the battle, the fortress celebrates even as Violet grieves her mother. Xaden, bloodied at the ravine, says he killed the Sage and urges a return as Aretia’s wards weaken. A faint red ring rims his eyes—an ominous change. In a final coda, Xaden recalls being bound by the Sage, reaching for a subterranean current despite Sgaeyl’s warning, and later slips to Jack’s cell in secret. Jack claims there is no cure for what he has become—only control—and welcomes him “as a brother,” leaving Violet’s victory shadowed by a new, intimate threat.