Shortly after the crux migration devastates the city of Ellder, capital of the kingdom of Turah, Ransom arrives at Treow—the Turan encampment—to find it deserted. His traveling companion, Cathlin, a woman with stark white hair, produces two bloodstained knives belonging to Ransom’s wife, Odessa, found abandoned in the street. A caretaker named Mariette confirms that soldiers sent by Ransom’s father, King Ramsey, came before dawn and led the encampment toward the hidden capital, Allesaria—but Odessa and Ransom’s young sister, Evie, were not with them. Ransom and Cathlin remark that the crux shapeshifter they saw resembled Odessa. Ransom worries that his father, whom he characterizes metaphorically as a monster, has found Odessa and Evie. He swears that he will kill his father if he hurts them. He sends Cathlin back to Ellder to bribe the guards and any surviving witnesses to keep silent about the shapeshifting crux and to watch over the gravely wounded Zavier, Evie’s father. Believing Odessa would have taken Evie somewhere safe from his father, Ransom rides for Quentis.

Five days after fleeing Ellder, Odessa sits by a lake in the Turan wilderness with Evie asleep in her lap and her pet tarkin, Faze, nearby. The Voster priest Brother Dime arrives and insists they leave, followed shortly by Brother Skore with a spare horse. Dime reveals that they are heading to Ozarth, which alarms Odessa. A wordless look between the priests makes her fear that Ransom may be dead; Dime says only that their paths must divide because Ransom is loyal to the High Priest, and that this journey concerns Odessa’s dynasty. He invokes a promise made to her father. Odessa recalls that Ransom made her promise that if anything happened to Zavier, she would take Evie to far-off Quentis to protect her from King Ramsey. She pushes for a coastal route to Quentis per Ransom’s instructions, but Dime makes clear she has no choice—using Evie’s presence to enforce compliance.

On the continent of Kenn, Caspia wakes from a recurring vision—now her sixth—of her sister Emery being killed by a silver-eyed warrior. Her cousin Xandra persuades her to tell their aunt, Queen Oleana, for the sake of Emery’s young daughter, Graciella. Caspia is a descendant of a royal line of female shapeshifters called the Starling, who rule the kingdom of Nelfinex; Caspia and Xandra are Quiescents, those who have not yet undergone the ritus, the mystical transformation. Recently, two Quiescents have failed to manifest the gift of shapeshifting, something that has never happened before. Oleana, the queen who raised Caspia after her mother abandoned the family, worries that the gift is weakening. She dismisses the vision as a dream and forbids Caspia from speaking of it, warning that now is not the time to show weakness, as the king of neighboring Beesan is looking for an opportunity to invade. After the queen leaves, Caspia feels a powerful pull toward the sea—the call of the ritus—and understands that it is connected to Emery’s fate.

Eight days after leaving Ellder, Odessa rides through a dark forest at night with Evie asleep in front of her, flanked by both priests. Evie wakes from a nightmare crying for her father, and before Odessa can comfort her, a nearby roar sends them into a hard gallop. They reach a remote cabin, where a couple named Damon and Sally welcome them inside. Over dinner, Damon tells Evie a story about a lionwick claw necklace—Odessa recognizes it from a journal belonging to Evie’s mother, Luella. She believes that the journal led Luella to create the elixir that gave rise to Lyssa, a deadly infection that makes monsters more powerful and aggressive. After Evie is asleep, Odessa takes out the leather journal and resolves to learn the old language it is written in, believing it may hold a cure for Lyssa.

In the days after confronting Oleana, Caspia secretly prepares to leave Nelfinex, packing supplies and leaving farewell letters behind. To pass the palace sentries, she wears her missing sister Emery’s elfalter rings—symbols of full Starling status—stolen from Oleana’s room. Emery had abandoned the rings when she fled with a forbidden Beesan partner named Max after Oleana forced her to choose between him and the throne. At the docks, a pirate captain named Cap—Max’s brother—is waiting. Caspia pays him with her elfalter armbands for passage. As she boards, she sees Oleana circling the palace in the form of a swift, screaming. Caspia silently vows to avenge Emery and sails into the dark.

The following morning, Odessa reviews Luella’s elixir recipe, her notes toward a Lyssa cure, and the journal in the old language. She also studies Ransom’s leather cuff, which maps the secret road to Allesaria—the hidden Turan capital her father had sent her to find as his spy—and decides to keep her own copy unfinished until she understands his motives. When Evie appears in her bloodstained shirt from Ellder and refuses to change, Odessa is forced to remove it, triggering an intense emotional breakdown. Brother Dime then announces he is leaving to meet the High Priest; Skore will escort them forward and teach Odessa the old language. After Dime departs, Odessa notices that she cannot feel Skore’s magic. Suspecting that Skore is weaker than Dime, she begins planning an escape.

At sea aboard the pirate ship Cirrina, Caspia stands at the bow as the pull of the ritus draws her forward. A stowaway is discovered: Xandra, who paid Cap with all her elfalter armbands. Xandra admits she does not yet feel the ritus but has come to help seek vengeance for Emery. When crewmen spot two large sea creatures called marroweels, Caspia and Xandra simply extend their hands, and the creatures approach calmly, subdued by their Starling bloodline. Privately, Caspia tells Xandra she hired Cap because he is Max’s brother, hoping for information about where Emery went.

Riding through Ozarth, Odessa practices the old language with Skore and reflects on the many secrets she keeps from Evie. Evie believes that her parents are Odessa and Zavier, but the truth is much more complicated: Odessa’s husband is not Zavier but Ransom, Zavier’s nearly identical cousin; Evie’s biological mother is Luella, whom she knows as a caregiver and teacher, and her biological father is Ramsey, the cruel king of Turah. She is Ransom’s sister. The group follows smoke to find a gray-haired woman using a folk remedy for a snakebite—a scene Odessa recognizes precisely from Luella’s journal. Skore hauls the woman to safety and encases her burn in ice, a power Odessa had only seen from the High Priest. Shortly after, a pack of five bariwolves—huge, half-furred, half-scaled monsters—traps Odessa and Evie against a ledge. Skore then draws the blood from all five creatures through the air until they collapse as lifeless husks. The final drops are green: The wolves were infected with Lyssa, proving that the disease has spread into Ozarth.

Many days into the voyage, Cap threatens to turn back as rations run low. Caspia talks him into one more day. Xandra reveals that she has finally begun to feel the ritus—a thrumming like a second heartbeat, confirming that they are heading the right way. Caspia shares two recent visions she cannot interpret: a boy making books float in the air, and a hunter who frees a golden cat from a trap and receives its severed claw as a gift, granting him lifelong health. She then sleeps and has another vision of a broad-framed man and a gray-eyed girl laughing together on horseback, riding toward blue mountains. The thrumming surges to an overwhelming peak and then stops as a crewman shouts: land.

At camp after the bariwolf attack, Odessa learns that they are heading to Orson Canyon. Four days later, they enter the narrow, eerie gorge—and Odessa’s ability to feel Skore’s magic disappears entirely. He leads them along a ledge beside a massive waterfall and into a hidden cave behind it, where he asks for her help retrieving something from a tunnel, since she can sense magic. Inside, the walls glow with iridescent blue veins. Following Skore’s instruction to pursue the sensation of pain, Odessa navigates a maze of passages guided by intensifying magic, growing increasingly ill. At a dead end, she kneels beside a small black pool. When her fingers graze the water, a wave of searing agony shoots up her arm. Skore praises her and touches her hand, and she faints.

Two days after landing on an unknown continent north of Kenn, Caspia and Xandra say farewell to Cap’s crew on a sandy beach. They follow a stream inland through vivid green hills but find no people, roads, or towns, and the ritus call has gone quiet. Both grow progressively unsteady, which they attribute to weeks at sea. As evening nears, a storm breaks, and they shelter in a large cliff cave lined with strange, tapering stone columns. Wet and exhausted, they settle in for the night.

Two days later, Odessa wakes in a grove of trees. Evie is unharmed and tells her that she has been unconscious the entire time. When Odessa presses Skore about the tunnel, he deflects, then announces that his part of the journey is over. He gives her directions to a town called Norcrest and instructs her to find guides in the bogs who can lead her to Quentis. He warns that her father’s life depends on keeping the waterfall a secret, cautions her to stay away from the High Priest, and tells her to seek out a warrior who will save Calandra. Before departing, he reveals he can suppress his own magic at will. After he rides away, Evie mentions that she saw him quietly retrieve a round, clear, melon-sized object from his bag the previous night. Suspecting that she helped him find it in the tunnel, Odessa breaks camp immediately and heads south, now alone and unprotected.

Three days into their time on the unknown continent, Caspia and Xandra can barely walk. Both have been sick since the cave—feverish, cramping, and in escalating pain—and Xandra’s eyes have turned milky white. Believing they are dying, they hold each other as they lose consciousness. Caspia is eventually awakened by a clicking sound and rouses Xandra. They flee down a forest road until three large jackal-like beasts surround them; Caspia injures her ankle in a fall. As the creatures close in, Xandra’s ritus begins—her eyes go black and her teeth lengthen into fangs. She orders Caspia to run. Caspia flees but is cornered at a cliff above a raging river. A larger beast—Xandra—attacks and kills one of the others, and the remaining two submit to her. But Xandra then stalks toward Caspia with the same menace. As she leaps, Caspia jumps off the cliff.