Henri meets the Austrian messenger, Baron Wolfgang von Polhaim, on the road to Nantes. To ensure secrecy, Henri brings Polhaim to the castle for Eastertide revels and encourages him to drink. A drunk Polhaim flirts with a charming young woman, unaware that she is Duchess Anne herself. Later, Henri retrieves Polhaim from the party for a secret late-night meeting with Anne and her council. Anne reveals her daring plan: She will marry Maximilien of Austria by proxy in a chapel within Brocéliande during the unicorn hunt. She explains that because French diviners cannot see into the forest, the marriage will remain a secret, giving Maximilien’s army time to cross the border and defend Brittany before France can react. Though surprised by the ingenuity and complexity of the Duchess’s plan, Polhaim agrees to ride back to Maximilien at dawn to secure his approval.

After the meeting, Jean warns Anne that her plan is dangerous, especially for her younger sister, Isabeau, who will be forced to marry the French king herself if it fails. Later, Isabeau confronts Anne, distressed by court rumors that Anne will marry the French king and be forced to leave Brittany forever. Anne reassures Isabeau that she is secretly planning a different marriage to protect their realm, but admits that she will have to leave to live with her new husband when the time comes.

To comfort the distraught Isabeau, Anne tells her the traditional Breton legend of Keris, a magical city that was lost to the sea. She recounts the story of its king, his magical wife from the fairy-folk known as the korriganed, and their willful daughter, Ahèz, whose actions led to the city’s downfall.

Baron von Polhaim returns to his master, Maximilien of Austria, who agrees to Anne’s plan for a secret wedding. Maximilien designates Polhaim to act as his proxy and sends him directly to Brocéliande, dispatching acceptance to Anne via diviner rather than routing Polhaim back through Nantes. Meanwhile, Anne’s hunting party departs from Nantes. On the journey, Anne rescues a young orphan girl named Elesbed from a ruined farmstead. Anne’s personal retinue arrives ahead of the main party at the convent of Paimpont in Brocéliande to await Polhaim. There, Anne is summoned to a private meeting with the abbess in a room strangely illuminated by moonlight, despite the storm outside. The abbess questions Anne’s motives intensely until Anne reveals the wedding plan. Shortly after their strange conversation, Polhaim arrives with a bishop. Anne sees the storm outside and the pitch-black sky, and realizes the moonlight in the room couldn’t have come from the moon. When she turns back, the abbess she spoke with has vanished.

In the convent’s eerie chapel, Anne is married to Maximilien of Austria, with Wolfgang von Polhaim standing in as the legal proxy. The ceremony is interrupted by the appearance of a ghost—a spectral woman the Bretons call an anaon—who is dressed in antique clothing and seems terrified of being killed by her husband. The ghost gives Anne a small, heavy, glittering object, pleading with Anne to keep it safe from him, saying only that he ruins everything with it. Just as the ghost vanishes, Anne witnesses four spectral women: A rose-silk ghost with a slashed throat, one drowned-blue, one hanged-black holding her own rope, and one engulfed in flames.

Following the wedding, Anne and Polhaim perform the legally required proxy-consummation ceremony before witnesses, in which Polhaim, wearing full armor with his right leg and right hand bared, touches his bare right leg to hers in bed. Anne decides to keep the ghost’s object a secret.

The morning after the wedding, Anne’s nurse, Hawiz, locks the mysterious object away in a coffer for safekeeping, referring to it as a mirror. The orphan girl, Elesbed, startles Anne by claiming she has found her doll, which was lost back at the ruined farm, explaining that lost things often reappear near the enchanted Lost Lands.

When the mother superior appears, Anne realizes she is not the abbess she met the previous night. Unsettled, Anne proceeds with her plan. She bids farewell to Polhaim, who departs with the signed marriage documents to signal Maximilien to move his army. As a token for her new husband, Anne gives Polhaim her glove. Shortly thereafter, the full hunting party, led by the unsuspecting La Trémoille, assembles to begin the hunt for the unicorn.

Anne, dressed in white, acts as bait in a clearing. To everyone’s astonishment, a real unicorn appears, chased by a second, mysterious hunting party that is only visible in strange patches of sunlight. When La Trémoille’s party spots the unicorn and gives chase, the creature flees but stays close to Anne. During the pursuit, Anne and the unicorn seem to shift between the normal world and the sunlit world of the Lost Lands. To escape both hunts, Anne deliberately rides her horse into a lake; the unicorn hesitates before following, and the crossing is nearly fatal as Anne’s horse, Jonquil, struggles under waterlogged skirts and the unicorn fights to keep her horn above water.

Once they are safe on the other side, the unicorn allows Anne to approach and cut a lock of its mane before it disappears into the forest. A man in archaic clothing stumbles out of the woods, claiming he has been trapped in the Lost Lands for 200 years. When La Trémoille’s party arrives, Anne presents the lock of unicorn mane and declares the unicorn a holy creature that should never be killed. La Trémoille disagrees, but the hunt ends because the hounds have lost the scent.