Sebastian, whose mother died on the day he was born, lives with César, the patriarch of the mountains, along with Angelina and Jean, César's grandchildren. Wild and freedom-loving, the boy dreams of a large dog, terrified by its confinement in a kennel, that has escaped. The whole village fears the enormous animal. Only César understands the boy, and together they decide to set out in search of the dog.

Angelina is in love with the doctor, Guillaume, who comes to join her at the chalet while César takes Sebastian to the Grand Baou refuge. There, the old man tells the child that he was born there and how his mother died giving birth to him. Seeing the growing concern in the village about the large stray dog, Guillaume confides his worries to César. One morning, starving, Belle comes down to the village, spreading fear among the villagers, who decide to organize a hunt. Two men from the village ask César to join them in tracking the beast. The old man promises to take up his rifle, but not against the large stray dog, against the men who are hunting it.

Sebastian sets off into the mountains in search of Belle. He finally finds her. Exhausted but happy, the boy returns late to the chalet, telling César about his encounter. In the village, the men organize the hunt. Guillaume tries to reason with them, but in vain. Arriving near the Grand Baou refuge, the village men are stopped by César, who invites them to follow him. They are astonished to discover Sebastian playing with Belle. From then on, Belle becomes Sebastian's friend, and they are inseparable. In the village, people are still a little frightened by this large white animal. So, when the mayor suggests returning Belle to the kennel, Sebastian firmly opposes it.

Norbert, a foreigner who came to the dam near the village looking for work, is staying with César. At the chalet, no one welcomes the newcomer except Jean. Jean falls under Norbert's bad influence, as he doesn't hesitate to get him drunk to extract information about the family. Jean confides in César about Norbert's plan: to train Belle to be a good guide dog for tourists. César warns Jean about Norbert. Taking advantage of César's departure to hunt in the mountains, Norbert tries to seduce Angelina, who violently rejects him. He then leaves to catch the train to Castellane to retrieve a suitcase he supposedly left with his former boss.

Norbert walks down towards the valley. A car is waiting for him on the road. Sebastian, leaning on Belle, watches in surprise as the car drives away. Norbert lied: he did indeed go to Castellane, but to meet up with a gang of criminals he belongs to. Together, they devised a plan to illegally transport goods to the Italian border. Back at the chalet, Norbert announces to everyone that he will leave after Christmas. Sebastian confides in César that he saw Norbert get into a car and, that evening, get out with a suitcase. But they are far from suspecting that the suitcase contains a radio transmitter... César knows that Norbert and Jean are training Belle to go as far as the Grand Baou refuge. This worries him, and he keeps an eye on them. But he is not the only one: these comings and goings so close to the border have alerted the customs officers.

Why does Norbert make the trip from the chalet to the abandoned mountain cabin so often? Belle follows him obediently. Sebastian, like his dog, seems to sense danger. Norbert then decides to accelerate Belle's training...

Sebastian is becoming a nuisance to Jean and Norbert. So one morning, Jean doesn't hesitate to lock him in the barn to get rid of him. Norbert is persistently courting Angelina. Guillaume is itching to give him a good thrashing. But Angelina doesn't want any conflict, because of Jean. Meanwhile, Jean is having trouble with Belle, who refuses to go through the Grand Défilé. He's stopped and reprimanded by the customs officers, who send him back to the chalet. Norbert is furious and realizes he has to act fast. In the meantime, Belle escapes and makes it all the way to the Grand Baou refuge on her own. César, who's keeping watch, is sure he saw a man waiting for her there. But when he rushes into the refuge after the dog leaves, he finds no sign of anyone...

On Christmas Eve, the atmosphere is gloomy at César's house. Sebastian feels neglected. Taking advantage of Angelina's departure for the village, the young boy decides to hide Belle in the barn, but Norbert quickly discovers his deception. He frees the dog, who then sets off again for the Grand Parade with Jean. In the village, Guillaume declares his love for Angelina and asks her to marry him. Back at the chalet, Angelina can't find Sebastian: he has run away into the mountains and joined Belle. The customs officers, aware that the dog could be a dangerous tool for smuggling, decide to scare Sebastian, because in their minds, the boy is perfectly capable of keeping the dog on the farm if he wants to. They tell him that they will shoot Belle the next time they see her near the shelter. However, for Norbert, the training is complete, and the big moment is approaching...

Thanks to his radio transmitter, Norbert informs his contacts that the mission will take place tonight. At the chalet, Christmas Eve is sad and silent: Guillaume is held up in the village due to an emergency, and César is still in the mountains. At the customs post, the customs officers learn that a secret document has been stolen from the National Research Center and that a gang is trying to smuggle it across the border. During the night, while everyone is asleep, Norbert places the microfilmed document inside Belle's collar, a gift for Sebastian. Belle immediately wears her new collar for the first time and sets off into the mountains. Sebastian's cries and protests are in vain, and without a word, he too runs off into the mountains in search of Belle. At the Grand Baou refuge, accomplices are waiting for the dog. Their mission is to take the contents of the collar and cross the border through a cave with a secret entrance built into the refuge's chimney.

In the fog, it wasn't Belle the customs officers had injured, but Sebastian. Guided by Belle, Guillaume found the unconscious child and brought him back to the village to care for him. He entrusted him to Célestine while he returned to the chalet to let them know the child was safe there. Meanwhile, César and the customs officers rushed to the chalet to look for Norbert. But Norbert had fled, setting fire to the storeroom to destroy any evidence of his involvement in the microfilm affair. Jean, busy putting out the fire with the help of Guillaume and the customs officers, had no idea that the two men were there to arrest him and that the police believed him to be Norbert's accomplice.

Norbert, pursued by the police and riot police, runs through the mountains. As he prepares to use the secret passage in the Grand Baou refuge, he is stopped by customs officers accompanied by César. The old hiding place is soon discovered, containing a radio transmitter and the passage to Italy. Questioned, Norbert denies everything and accuses Jean of having smuggled the microfilm abroad himself. At the chalet, once the fire is extinguished, the police discover Norbert's transmitter. Before the eyes of a distraught César and a weeping Angelina, they arrest Jean. To protect Belle and fearing she will be killed, Sébastien flees with the dog to the Grand Baou refuge via the avalanche chute. He discards the dog's collar containing the microfilm, unaware that he is throwing away the only proof of Jean's innocence. In the mountains, the wind and fog are rising. Belle senses the danger. An avalanche is triggered...

Angelina and Guillaume find Sebastian numb from the cold but safe, and all the villagers who wanted Belle dead thank him for saving the child. Commissioner Leduc arrives in the village to conduct his investigation. Norbert maintains his accusation against Jean. The young man proclaims his innocence but has no proof. They would need to find the microfilm. However, all that is known is that it was in the chain Belle was wearing, but that she no longer has it...

Commissioner Leduc continues his investigation into the disappearance of the microfilm. He releases Jean and places him under the care of customs officer Joano. Sebastian confesses to Jean that he was the one who threw the chain into the ravine. Firmly resolved to prove his innocence, Jean flees and heads into the mountains in search of the chain. But while climbing the rock face, he falls and breaks his arm. Guillaume arrives to the rescue and manages to find the chain in the icy water of the stream. The microfilm was indeed hidden there. Jean is finally exonerated.

Sebastian, now nine years old, still accompanied by Belle, meets up with Pierre Maréchal, his biological father, owner of a stud farm in Provence. But the reunion is difficult, especially since Pierre is riddled with debt and has many other worries on his mind...

Pierre Maréchal, back home at the "Les Jonquières" stud farm, would like to tell Sylvia, his fiancée, about this unexpected son. But she's away on a trip. So he confides in old Thomas, the stable boy. Belle frightens the horses; when Thomas asks him to lead her, Sebastian wants to leave Les Jonquières...

Upon learning of Sebastian's existence, Sylvia forces Pierre to make an impossible choice: her or Sebastian. She breaks off their engagement. Back at "Les Jonquières," a despairing Pierre lashes out at everyone, especially Belle, whom he has locked up. Heartbroken, Sebastian realizes he is not well accepted...

Ordered by his father to apologize for hitting a stable boy, Sebastian is locked in a horse stall despite Thomas's pleas. César has decided to go to Canada for his grandson's wedding. Sebastian is sad, despite the support of Jeannette, his new friend...

At Sebastian's request, old Célestine arrives at "Les Jonquières" to replace the cook who resigned. Some owners are removing their horses from the stud farm. Pierre Maréchal is at his wit's end and flies into a violent rage in front of Thomas, who has come to plead Sebastian's case. Sebastian has overheard everything and has run off to the beach, where his father finds him sobbing uncontrollably...

Old Thomas teaches Sebastian to ride a horse. While walking on the beach with Belle, he meets Sylvia, who abruptly leaves him upon learning his true identity. Lambert explains to Bertrand how he can ruin Maréchal. Maréchal is forced to sell Lancelot. He is left with only Monseigneur, to whom Sebastian confides that he loves his father, even though his father doesn't understand him...

Pierre Maréchal resumes training Monseigneur. Through the notary Biard, he learns that it was Lambert who bought Lancelot. Sebastian tries to escape to join César in Canada...

Every morning, Sylvia gives Sebastian riding lessons in the woods, and he becomes an excellent rider. Bertrand, Sylvia's cousin, consumed by jealousy, asks a former stable boy from "Les Jonquières" to drug Monseigneur, who is to participate in the Prix du Marais...

Monseigneur was injured during the race; Sebastian stayed by the horse's side day and night to care for him. Pierre Maréchal sold everything he could to pay off his debts and keep "Les Jonquières," which he converted into a farm. While talking with Sylvia, Sebastian discovered that it was probably Bertrand who drugged Monseigneur...

Pierre Maréchal doesn't want to jeopardize Monseigneur's chances against Lambert's well-trained stable and refuses to enter him in the traditional July 14th gallop. He doesn't know that his son has become an excellent jockey...

After the race, it's time for the Bastille Day dance. Pierre Maréchal, following tradition, opens the ball with Sylvia. The rhythm of the waltz carries them away from the circle of dancers and they flee into a village alley, under Bertrand's furious gaze...

In Fontenelle, Bertrand and Sylvia's engagement is being celebrated. Raymond comes again to demand the promised money. Bertrand advises him not to complain because he would be accused of doping. Raymond then reveals everything to Sylvia, who runs away and goes to find Pierre Maréchal, to whom she tells how the Bishop was drugged...

Bertrand realizes his revenge is impossible and that he's been found out. He jumps in his car and flees, pursued by two motorcyclists. Running out of gas, he has to abandon his vehicle for an old clunker. But further down the road, a wrong turn sends him plunging into a ravine...

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.