Set in a fictional world resembling 19th-century France where some people possess telekinetic abilities, the story follows three characters caught in a web of love, ambition, and deception among the aristocratic elite of the capital city, Loisail.
Hector Auvray, a world-famous telekinetic performer who has spent the past 10 years abroad, returns to Loisail to reunite with Valérie Beaulieu, the woman he loved in his youth. At a high-society ball, he learns Valérie is ill and will not attend. Retreating to the library, he meets Antonina "Nina" Beaulieu, a young woman from rural Montipouret spending her first Grand Season, a months-long social calendar of balls and events, in the city. Nina is candid and unpolished, and Hector, charmed despite himself, dances with her three times. He later learns she is a cousin of Gaétan Beaulieu, the wealthy man who married Valérie. His friend Étienne Lémy, who knows about Hector's past with Valérie, warns him to stay away.
Nina lives under Valérie's supervision during the Season. Valérie, born into the impoverished noble Véries family, married Gaétan for his wealth rather than love and considers Nina unsophisticated. Nina possesses an uncontrolled telekinetic talent that has earned her the nickname "the Witch of Oldhouse" back home. Hector deliberately seeks Nina out and secures an invitation to the Beaulieu household, using the courtship as a pretext to be near Valérie.
When Hector sees Valérie again, their backstory emerges: They met 10 years earlier in the town of Frotnac, fell in love, and became secretly engaged. Hector left for Iblevad to make his fortune, but under pressure from her grandmother, Valérie ended the engagement with a three-line letter and married Gaétan. She has kept the cheap gold ring Hector gave her ever since. Though she agrees to let Hector visit, she feels compelled by a force she cannot resist.
Hector begins formally courting Nina: bringing her lilies, taking her to his show at the Royal theater, and teaching her to control her talent through hand movements and focus techniques. Their lessons become a source of genuine connection, and Gaétan grants Hector permission to court Nina. Yet Hector's heart remains fixed on Valérie. In a bitter private exchange, Valérie reveals her family forced her into marriage; Hector accuses her of never truly loving him.
When the Grand Season ends, Nina invites Hector to Oldhouse, her family's country estate. Surrounded by Nina's large, boisterous household, they grow closer, skipping stones, collecting insects, and exploring the library where Nina keeps her cabinet of pinned beetles. During a game of hide-and-seek, Nina leads Hector up an old tower, and they kiss. Feeling he has betrayed his loyalty to Valérie, Hector retreats. That night, Nina writes him a love letter and slides it under his door. Valérie intercepts it.
The next morning, Valérie confronts Hector and breaks down, admitting his pursuit of Nina has been like "a nail in my heart each day." He offers to take her away, but she refuses. She reveals she married Gaétan precisely because she was ready to throw everything away for Hector and could not allow anyone that power over her. Overcome, Hector kisses Valérie. Nina walks in and sees them. Her telekinetic power erupts: Books fly off shelves, and as she flees through a hallway of stained-glass windows, she shatters every panel. That night, Valérie threatens Nina with the intercepted love letter, warning she will use it to destroy Nina's reputation if Nina reveals what she witnessed. Crushed, Nina agrees to keep silent to protect Gaétan.
Hector leaves Oldhouse two days early, unable to speak with Nina, who refuses to open her door. On the train, he tells Étienne that Valérie's confession has destroyed his obsession and that Nina "wanted nothing from me . . . nothing at all but to let her love me." Nina retreats into months of isolation, but gradually she begins practicing the card tricks Hector taught her, shuffling them while repeating his name until the word loses its power. By winter, she regains control of her talent and resolves to return to Loisail.
Back in the city, Nina stays with her great-aunts rather than with Gaétan and Valérie. She encounters Luc Lémy, Étienne's younger brother, and they begin spending time together. Hector sends Nina 20 rare pinned beetles for her missed birthday. She confronts him at his apartment, furious, but he insists his affection was genuine and describes his obsession with Valérie as a sickness he could not stop. They cautiously rebuild their friendship. At the aquarium, Hector tells her his fixation on Valérie is over.
Behind the scenes, Valérie and Luc forge an alliance. Luc wants to build a luxury hotel on Véries family land and needs Nina's fortune to finance it. Valérie supports his courtship in exchange for a share of Nina's dowry and manipulates Gaétan into approving the match. When she learns Hector is still seeing Nina, Valérie visits him and offers herself as a lover if he stops. Hector refuses. Valérie then shows Gaétan Nina's intercepted love letter and pressures him to push the marriage forward. She lies to Nina, claiming Hector has recently pledged his love to Valérie again.
Devastated, Nina accepts Luc's proposal. When Hector intercepts her on the street, she tells him it is too late. At the engagement party Valérie orchestrates, Nina wears an aquamarine gown but feels as though she is drowning. Stepping outside for air, she follows a blue lightning bug, a warm-climate beetle that should not be in the city, through the dark streets. Losing sight of it, she realizes she wants Hector. She hails a carriage and rushes to his apartment, declaring she cannot marry Luc and has loved Hector since Oldhouse. Hector, terrified of being hurt, tries to send her back. Nina calls him a coward. He confesses she is already inside the box where he has locked his heart. They kiss, and Nina removes her engagement ring.
Luc arrives the next morning, challenging Hector to a duel with pistols at 20 paces. Valérie secretly urges Luc to shoot to kill. At dawn on the Lawn behind Clocktower Hill, both men fire. In the fraction of a second before the bullets hit, Nina, who has raced to the field, stops both in midair with an unprecedented telekinetic force and lets them drop to the grass. Valérie demands the pistols be reloaded, but Nina appeals to Luc directly. Moved, Luc declares the duel satisfied and confesses he pursued Nina partly for her money and that Valérie conspired to have Hector killed. Valérie begs Hector for a word of love. He says, "No. I'm sorry."
Gaétan banishes Valérie to a remote town near the northern border. On the train, she clutches Hector's old gold ring, then throws it out the window and immediately regrets it. Hector asks Gaétan for Nina's hand and receives it. The couple marries at Oldhouse in a modest summer ceremony. Hector reveals they will board a ship to Port Anselm the next day for a months-long journey across Iblevad. That night, he reflects that he drifted toward Nina from the beginning, "magnetized, a compass that had spun wildly and then gently settled upon a true north." He asks her to keep finding him. She turns from the window, flowers in her hair, and smiles.