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.