10-year-old Sébastien reluctantly spends his holidays in the mountains with his grandmother and aunt. He has to lend a hand at the sheepfold, nothing very exciting for a city boy like him... but he didn't count on meeting Belle, a huge dog mistreated by her master. Ready to do anything to avoid injustice and protect his new friend, Sébastien is about to experience the craziest summer of his life.