The first book in the Sevenwaters trilogy is set in early medieval Ireland, at Sevenwaters, a remote lakeside fortress deep within an ancient forest. Sorcha, the narrator, is the youngest of seven children born to Lord Colum, a chieftain consumed by a generations-old feud with the Britons over three sacred islands. Sorcha's mother died giving birth to her, and Colum, devastated by grief, withdraws from his children to devote himself to war. The six brothers raise Sorcha in a half-wild childhood. Liam is a disciplined leader; Diarmid is charming and brave; the twins Cormack and Conor are warrior and scholar; Padriac has a gift for animals and invention; and Finbar possesses the Sight, an ability to perceive what others cannot. The siblings communicate mind to mind, a gift that binds them to one another and to the forest's spiritual world. Stung as a child by the starwort plant, Sorcha teaches herself herb lore and healing, a vocation central to her identity.
Before the crisis that drives the plot, Sorcha and Finbar defy their father by secretly freeing a young Briton prisoner named Simon from torture. Sorcha later nurses Simon back to health at the cave of Father Brien, a Christian hermit, until her brothers arrive with urgent news: Colum intends to marry Lady Oonagh, a beautiful woman whose enchanting manner conceals something deeply sinister. Sorcha senses the danger at once. Oonagh systematically poisons the household, ensnaring Diarmid in an infatuation bordering on enchantment, poisoning the food of Liam's betrothed Eilis to drive her away, and destroying Sorcha's herb garden. One by one, the household's protections are dismantled.
Recognizing that Oonagh means to destroy them, the siblings gather at dawn by their mother's birch tree to summon the spirits of the forest. Oonagh intercepts them, binds the brothers in enchanted mist, and transforms all six into swans. Warned by Finbar's mental cry, Sorcha escapes into the forest. The Lady of the Forest appears to Sorcha. A powerful being of the Otherworld, the supernatural realm beyond the human world, the Lady explains the only way to break the spell: Sorcha must spin, weave, and sew six shirts entirely from starwort fibers. From that moment until the shirts are placed over her brothers' necks, she must not utter a single sound or communicate her story in any form. Breaking the silence means the curse remains forever. The brothers will return to human form for one night at midsummer and midwinter. Sorcha finds Father Brien dead in his cave, gathers what supplies she can, and walks into the forest alone.
Sorcha establishes a hidden refuge in a cave by the lake. The starwort's barbs embed themselves in her flesh, causing her hands to swell, blister, and bleed. She endures months of isolation, hunger, and cold, telling herself silent stories to stave off madness. When her brothers return at midsummer and midwinter, they are gaunt and disoriented. Finbar grows increasingly troubled by visions he will not share, and before one departure he places their mother's amulet around Sorcha's neck in a gesture of terrible finality. At the next midsummer, men assault Sorcha and kill her dog Linn. Her brothers arrive at dusk and kill the attackers. Finbar shields Sorcha's mind all night with beautiful memories, but the effort nearly destroys him. The Lady of the Forest tells Sorcha she must leave. A boat carries her downriver; when it capsizes, she is rescued by a red-haired Briton called Red.
Red is Lord Hugh of Harrowfield, Simon's elder brother. He finds Simon's carved token among her belongings, but she cannot speak. Red brings Sorcha with his group as they flee Irish warriors. In a sea cave, the Fair Folk, faery beings of the Otherworld, appear. The lord and lady of this supernatural court address Red by his true name, taunt him with knowledge of Simon, and warn him to protect Sorcha. The group sails to Britain. At Harrowfield, Red's prosperous estate, Sorcha is received with hostility by his mother Lady Anne and the household. Red declares her his guest and stations guards outside her door. She resumes spinning starwort amid whispered insults. Only Margery, the wife of Red's companion John, befriends her, extracting barbs from her hands each day.
Red's uncle, Lord Richard of Northwoods, has long planned for Red to marry his daughter Elaine, uniting the estates under Richard's control. Richard subjects Sorcha to menacing interrogation and predatory advances that Red forcibly stops. Danger mounts: John is killed in a rockfall while guarding Sorcha, and a fire destroys one of the half-finished shirts along with her spinning tools. On May Day eve, Red cancels his wedding to Elaine and proposes a marriage of protection: As his wife, Sorcha will be safe while he travels to find Simon, who may be alive in a monastery off the Irish coast. Sorcha accepts, and Red gives her a ring carved from oak heartwood before departing by sea.
In Red's absence, Richard assumes control of Harrowfield. When Sorcha slips away at midsummer to embrace her brother Conor, who has briefly returned in human form, Richard and Red's companion Ben discover them and seize on the scene as proof of adultery. Sorcha is imprisoned. Richard visits daily, revealing his alliance with Eamonn of the Marshes, Eilis's intended husband, and with Lady Oonagh herself, who has demanded that Sorcha and her shirts be burned together. A formal hearing is arranged, but the presiding cleric, Father Dominic, is called away before delivering a verdict, and Richard announces a guilty sentence on his own authority.
Bound to a stake on the pyre, Sorcha sends a mental summons to Conor. As the sun sets, five swans circle the courtyard and land beside her. She throws the completed shirts over them. Red, Ben, and Simon burst through the crowd; Red has found his brother alive. Richard lights the fire. The sixth swan, Finbar, circles overhead. Sorcha spots an archer aiming at Red and screams to warn him, breaking her years-long silence. Wind carries the incomplete sixth shirt over Finbar's neck just before her cry. Red pulls the arrow from his shoulder, knocks Richard down, and leaps through the flames to cut Sorcha free. Five brothers transform into men. Finbar, whose shirt lacked a sleeve, retains a swan's wing in place of his left arm. Father Dominic arrives and declares the burning a miscarriage of justice.
Sorcha's brothers take her home across the sea. Red tells her she is free to go; when she tries to return his ring, he refuses. At Sevenwaters, the siblings find the estate in ruins and their father barely coherent. The brothers scatter, and Finbar vanishes, leaving only their mother's amulet and a white feather on the lakeshore. The Lady of the Forest reveals that Red was never enchanted; though Sorcha feared his devotion was compelled by magic, he loved her freely from the moment he pulled her from the water. Red arrives at Sevenwaters alone, having given Harrowfield to Simon, but is taken captive by Sorcha's family and brought before Colum blindfolded. Challenged by Colum to plead his case, he speaks of a woman who was "bone of my bone, and breath of my breath" and says he has left everything behind for her. Colum tells Sorcha the choice is hers. She removes Red's blindfold, kisses him, and chooses him. Together they begin restoring the estate, and Sorcha becomes pregnant. The novel closes as they ride home through the forest, carrying the promise of a new generation to inherit Sevenwaters.