Fourth Wing begins with Violet Sorrengail, the daughter of the powerful General Lilith Sorrengail, being forced against her wishes to join the dragon-riding Quadrant of the infamous Basgiath War College. Violet, who has spent her life training to be a Scribe due to her physical frailty and distaste for violence, must adapt quickly to survive an institution designed to eliminate the weak. Violet’s mother, General Sorrengail, insists that Violet must prove herself worthy in this brutal environment. Violet’s older sister Mira tries to help her by warning her to give a student named Xaden Riorson a wide berth. Xaden is the son of the rebel leader Fen Riorson, whom Lilith Sorrengail executed five years earlier; he likely wants revenge and has a reputation for violence. He and the other children of Rebels are called “the marked” because of the magical tattoos they bear, designating them as traitors to Navarre. Mira and Violet’s own brother Brennan died in the same Rebellion, so there is no love lost on either side.
Basgiath recruitment starts with a test where prospective riders must cross a dangerous parapet to get to the Riders Quadrant. Violet narrowly survives after watching Jack Barlowe, another Cadet, murdering a potential classmate by pushing them off. Violet offers Rhiannon, a classmate, one of her boots so she can survive the crossing, and the two become friends. When she arrives at the other end, she meets a former childhood friend and the son of her mother’s principal advisor, Dain Aetos. Having survived the parapet run, Violet and Rhiannon are recruited to Dain’s squad: within the Riders Quadrant there are four “Wings,” each of which contains three squads with three sub-sections. Violet then meets Xaden Riorson. After a very tense moment, Xaden declares that Dain’s squad will join the Fourth Wing, of which he is Wingleader. Violet is terrified because this puts her under his thumb.
As the semester at Basgiath begins, Violet learns about the longstanding conflict between her own home of Navarre and the neighboring kingdom of Poromiel. While Navarre’s army uses dragons to fight, the Poromiel army fly on gryphons, and they have been sending raiders out to cause havoc and rob Navarrian towns. The cadets all undergo intense, violent combat training. Violet gets crushed regularly but knows she cannot keep losing her fights, so she uses her Scribe’s knowledge of herbs to secretly poison the people she’s slated to practice against. The War College Rider training is structured around three major phases of training before qualifying Squad Battles and final War Games—the Gauntlet, the Threshing, and the Warfare course.
During the Gauntlet, cadets are physically tested in a deadly obstacle course that claims many lives every year; the overall survival rate for cadets is about 30%. The first-year squads all have to face this exam, which is a highly dangerous climbing challenge that features five obstacles. After completing the Gauntlet by the skin of her teeth, Violet has to face the Threshing. In this process, cadets must attempt to bond with a dragon. It is one of the riskiest trials, as dragons regularly kill and eat cadets during this time. Bonding with a dragon is crucial; without a bond, a Cadet cannot continue training. Violet defends a juvenile dragon from other cadets who try to kill it, and then unexpectedly bonds with both this dragon, named Andarna, and Tairn, an enormous and ancient black dragon. This dual bond is unheard of and instantly makes Violet a target. It also ties her inextricably to Xaden, as his dragon Sgaeyl and Tairn are mated. A rider dies if their dragon dies, and a rider’s death is also dangerous for the dragon. Xaden now must keep Violet alive for his own sake as well as Sgaeyl’s.
Violet and Xaden begin to psychically bond, which is a deeply unsettling experience for both of them. Violet, who has learned that Xaden has been holding illegal meetings of marked cadets, struggles to handle her own hatred of him and her undeniable attraction to him. The sexual chemistry between Xander and Violet is amplified by their dragon’s own highly sexed relationship. Led by Jack Barlowe, a group of cadets tries to kill Violet in an attempt to win a bond with Tairn, but Andarna saves her by freezing time. Xaden tells Liam, a large and intimidating Cadet and his foster brother, to watch over Violet. Violet begins to learn how to fly on Tairn. She and her squad win a competitive pitched battle, and as a prize are allowed to fly to a Navarrian outpost that Violet’s sister Mira is defending. Xaden goes with her, as Sgaeyl and Tairn can’t be separated for long or they’ll die. Sgaeyl and Tairn have sex. Because they’re psychically linked to their riders, this transfers those desires to them, which provokes Xaden and Violet into kissing.
Violet starts to try and train to use her “signet,” or special magical ability that riders gain from dragons. It doesn’t appear consistently, however, until the War Games, which come before the cadets’ graduation. In a game of Capture the Flag, Violet freezes with Andarna to try and save the life of her friend Liam. She’s unsuccessful, because his dragon Deigh has been killed. Furious, she uses her newly-discovered signet power from Tairn to call lightning down and kill Jack Barlowe. She begins trying to train to control it. Over this period Xaden and Violet eventually give into their sexual tension and begin sleeping together. Xaden is initially very reluctant to admit he is in a relationship with Violet, but after she tells him she won’t be with him unless he does, he eventually relents.
At the end of the War Games each squad of riders must complete a mission at one of Navarre’s remote outposts. Xaden’s squad are assigned to the most dangerous position, a remote fort called Aethebyne which lies outside of the territory protected by Navarre’s magical wards. Xaden brings Liam and Violet with him, but upon arrival it becomes clear that the squad has been betrayed and that the games have become very real. Xaden and Violet investigate and figure out that Dain has been using his own signet—the ability to read memories—on Violet to spy on Xaden and learn his plans. More shocking twists come thick and fast, as Xaden has been secretly helping the Poromelian gryphon riders, who, it turns out, are only attacking Navarrian outposts because they’re under attack themselves. Dangerous, wyvern-riding magical creatures called venin are sweeping over Poromiel. Violet, like most Navarrians, has always believed they were only the stuff of legends, but they’re real. She learns that the threat of gryphons and their fliers, long portrayed by the Navarrian government as the natural enemies of dragons and riders, has been manipulated to control the population and maintain power. This realization forces Violet to question everything she knows about her family’s role in this military oppression.
The novel concludes with a climactic battle where Violet, Xaden, and their allies must fight to protect a trading village that’s under attack by a group of venin. Violet, after a lot of bloodshed and painful trial and error, discovers that wyverns can be killed by killing the venin that conjured them. This wins them the battle, but Violet is poisoned by venin magic. She’s taken away to be healed, and when she wakes, she’s startled to be in the (she thought) destroyed Rebel stronghold of Aretia, to be with Xaden, and to be greeted by her brother Brennan. He’s not dead, after all, and he’s on the side of the revolutionaries against the corruption in Navarre and the venin. Violet has to decide where her loyalties truly lie, as the novel ends.