Set in the year 1945, against the stunning backdrop of Saint-Martin, a quaint village in the French Alps, this adventurous saga follows the fluctuating fortunes in the life of Sébastien, a nine-year-old orphan. He lives with César, his adoptive 'grandfather'. They await César's daughter Angélina, who, having fought with the Resistance until the end of the war, is coming home by plane. On the journey, Angélina's plane crashes in the Italian Alps, apparently with no survivors. César breaks the news to Sébastien, who refuses to accept she has perished.
César arranges for a local cantankerous pilot Pierre to make a fly-by of the crash site. César tells Sébastien that Pierre is probably his real father. Sébastien and Belle sneak on board Pierre's plane for the fly-by and see a flare lit near the crash site which Pierre does not. They argue and Sébastien grabs the joystick, causing the plane to land awkwardly and lose a wheel. The three of them are stranded in the mountains.
Sébastien and Belle start to walk towards the crash site in search of Angélina. Pierre eventually catches up with them, finding Sébastien has set up a camp and is quite confident in the forest. On the way they rescue Gabriele from a bear attack. She is a 14-year-old girl masquerading as a boy, because she is the only girl in an all-male Italian logging camp and her father fears for her safety. Another experienced hiker, she befriends Sébastien and leads them to the logging camp.
Sébastien again tries to escape to look for Angélina at the base of the cliffs, but is recaptured and locked up at the camp, while Belle follows a trail to the cliffs and eventually returns to camp. Pierre is still not convinced Angélina is alive, and angry with Sébastien for consistently disobeying him, but because by now he knows Sébastien may be his son, they have a rapprochement.
The camp awakes in the morning to find a bushfire threatening in strong winds, but Sébastien, Belle and Gabriele have left with supplies, again to follow Belle to where Angélina may be. They are caught by the bushfire and Pierre, again following them, saves them by extinguishing the fire with a stick of dynamite. They walk to the cliff base and enter a cave, following Belle. There is no sign of Angélina on the cave floor, but Sébastien is lowered by rope to a lower cave, where he finds Angélina, who had fallen to this lower level. She appears harmed but gets up with only a damaged arm, and is winched back to safety by Pierre and Gabriele. By now, the bushfire threatens the cave mouth so Belle leads them up a back exit to the top of the cliffs, and they narrowly escape asphyxiation.
They are next seen at the aircraft, which has been repaired with the aid of the Italian loggers and Gabriele's father. They all fly back to France, passing over César and Pierre's mechanic, who had walked together for two days to reach the crash site. César joyfully turns back for home, having seen a hand waving from the plane.
The final scenes see César arriving at his house in Saint-Martin; he embraces Angélina, as she emerges from hanging out the washing. Sébastien and Pierre sit at the grave of Sébastien's dead mother, who was carrying him at the time of her death in the first movie. It emerges Pierre loved her, but she was a Romany gypsy woman, so their affair was secret from everybody, particularly the male gypsy siblings. Pierre did not know she was pregnant at the time of her death. He had searched for her for months without success, after his own release from German prison camp. It then pans to a shot of the alpine valley in summer.