After the mountain setting, after the tracks trodden by racehorses, Sebastian is traveling through a completely new landscape on the train that takes him to the sea. To Brittany, where he will meet an uncle, Captain Maréchal, whom he doesn't know but who has asked to see him. A carriage awaits him at the station, driven by a dignified, somewhat stiff woman dressed in black. The journey that will take Sebastian to Morsan, his uncle's estate, will already give him a glimpse of the strange, even fantastical, atmosphere that surrounds him. Clarisse, the lady in black who drives the carriage, does everything she can to ensure this: she speaks of misfortune, of souls that wander after death, particularly that of Joseph Thépahnie... The first contact with his uncle will be somewhat distant. The boy remains wary. The captain has chosen him as his heir.

As Sebastian prepares to spend his first night in his uncle's manor, he hears unsettling noises above his room. Contrary to Clarisse's discouraging words, who claims a ghost is visiting the attic, Sebastian discovers the source of the commotion: a friendly dog ​​playing the harmonica. By day, the manor is much less ominous: Sebastian meets Lancelot, the horse his uncle has just given him. Through conversations with his uncle and the servants, the naive Sebastian gradually uncovers the secret surrounding the disappearance of the captain's wife and son.

As Sebastian speeds through the countryside on Lancelot's back, he accidentally encounters Sophie-Virginie, commonly known as Siza. The daughter of Captain Maréchal's business partner, she assists him in his affairs. Little by little, bonds of affection and trust develop between Sebastian and the old man. Yet, every conversation brings back the painful memory of Claire and Gilles' disappearance. Jonathan takes Sebastian to the port where they learn that the Narval, a ship belonging to the Maréchal and Walter shipping company, has been the victim of a serious accident caused by the shipowner's culpable negligence.

While chasing a dog that has stolen his shoe, Sebastian discovers a secret room in the manor's cellar, a bedroom that has been set up inside. Jonathan, the servant, then reveals the family's past as members of the Maréchal resistance. Jonathan takes Sebastian fishing in Morsan Bay and shows him the "Mary-Morgane." During the Second World War, this boat allowed resistance fighters to reach England. Joseph Théphanie, the boat's captain, was suspected of having denounced the Maréchal family to the militia, which led to the arrest and disappearance of Claire and Gilles.

At Morsan Manor, Sebastian continues his investigation. Although his questions irritate those he speaks to, the sympathy he inspires in them allows him to better understand the atmosphere of sadness that pervades the old building. Aboard the "Narval," anchored off the coast of Greenland, a fire has broken out. The local population is moved by the fate of the sailors and accuses Captain Maréchal, who refuses to relinquish command of the ship to the descendant of the man he believes is responsible for the deaths of Claire and Gilles.

On board the Narval, the situation is becoming increasingly alarming. The inexperienced captain is struggling to control the fire, which is claiming more and more lives. His son, Bernard, violently confronts Sebastian. Sebastian then reveals a much darker side of Captain Maréchal. When the damage is repaired and the crew is successfully rescued, tensions ease. Sebastian and Bernard bond over a bottle of cider, until they become intoxicated.

While the crew of the "Narval" seems to be out of danger, the inhabitants of Morsan Manor return to their daily gloom. Clarisse prays alone in a small church, Siza thinks only of Gwen Théphanie, whose return is delayed, and Captain Maréchal sails the seas to escape the sadness that has haunted him since his wife's disappearance. Sebastian joins his uncle and undergoes his initiation as a cabin boy, not without some mishaps...

The crew of the Narval finally returns to port to the cheers of the crowd. Sebastian and his uncle, for different reasons, cannot share in the jubilation for long; both resent Gwen Théphanie. Captain Maréchal harbors a deep grudge against the Théphanie family. Sebastian has discovered that the woman he was secretly in love with is Gwen's girlfriend...

The Narval's triumphant return makes Gwen Thephanie, its captain, a true local hero. The murky history of his family, suspected of collaboration during the occupation, fades in the hearts of the townspeople, who now see him only as a courageous ship's captain. Captain Marshal, more bitter than usual, no longer responds to the requests of his partner, Carl Walther. He decides to set sail alone, leaving Sebastian in Siza's company.

While Captain Maréchal is at sea, Sebastian finally gets the answer that has been nagging at him: why do the locals harbor such resentment toward the old shipowner? It is from Siza, Eugène (an old fisherman), and Bernard Lemoine that Sebastian learns how Captain Maréchal took revenge on the man he held responsible for the disappearance of Claire and Gilles Maréchal. The entire Théphanie family was targeted in this act of vengeance. Sebastian then discovers a dark side to his uncle, to whom he had until now felt a deep and affectionate loyalty.

Outraged by his uncle's past actions, Sebastian decides to leave the Morsan manor as soon as possible. As a storm plunges the old building into darkness, Captain Maréchal's health suddenly deteriorates. Sebastian runs to the village and alerts Doctor Sabelle, who rushes to the sick man's bedside.

Following his long discussion with Doctor Sabelle, Sebastian decides to postpone his departure, knowing that his presence is essential for his uncle's recovery. However, the old man's room is empty: barely recovered from a heart attack, the Captain has set sail in the middle of the night. Sebastian then sets off in pursuit.

On the "Mary Morgane," the captain takes his eyes off the course he's set to look around. He spots Sebastian's dinghy, tacks, and takes the child in. The wind picks up, a squall hits. The captain, worried by the opening of a hatch at the bow, wants to rush over. Sebastian sees him approach the railing, clutch his heart, and collapse. Sebastian rushes to him. Louis Maréchal will die relieved and happy, having accepted that Sebastian will be the one to settle the old score and the bearer of forgiveness for all. Louis Maréchal's funeral was extremely simple and quick. Sebastian stands up and begins to read Louis Maréchal's last wishes: "To you, Gwen, the Narwhal. Jonathan and Clarisse, you must get married. And you, Bernard, you must go to school..." Everyone is concerned. We look at each other, unsure whether to burst out laughing, worry about Sebastian's mental state, or pay attention to what he's saying, but Sebastian is vehement and ultimately persuasive.