Sebastian, a nine-year-old boy, lives in a small French Pyrenees village near Spain. Born during heavy snowfall on Saint Sebastian's Day to Romani mother Isabel, he was abandoned as an infant due to her community's disapproval of her marriage to an outsider. Raised by adoptive grandfather Cecil and his overprotective granddaughter Anne-Marie, Sebastian grows up unaware of his mother's true fate or whereabouts, though she promised to return.

He endures social isolation and bullying from village children because of his orphan status and Romani heritage. This loneliness defines his early years until he encounters Belle, a large Pyrenean Mountain Dog wrongly labeled "The White Monster" by locals. Recognizing her as a fellow outcast, he forges an immediate bond, names her Belle, and takes responsibility for her safety alongside his small dog, Poochie. As authorities intensify efforts to capture or kill Belle, Sebastian flees his village. He embarks on a journey across the Pyrenees into Spain to search for his mother and protect Belle.

Throughout his journey, Sebastian displays resilience, resourcefulness, and a strong moral compass. He confronts numerous challenges, evading persistent pursuers like Inspector Garcia and his clumsy assistant Officer Martin, as well as opportunistic criminals Hernandez and Fernandez who seek to profit from capturing Belle. Despite these dangers, Sebastian consistently aids others he encounters, helping in rescues and uncovering criminal activities like smuggling. His kindness and determination gradually win him allies, including Sarah, a wealthy but sick Spanish girl celebrating her ninth birthday during the series. Sarah becomes a close friend and eventual romantic interest, providing crucial assistance. Her initially dog-averse father, Mr. Albert, softens towards Sebastian and Belle after witnessing their positive impact on Sarah's health and happiness.

Sebastian's quest brings him into contact with various groups, including the traveling Romani performers of the Carlos Company, where his mother Isabel is a star performer and seamstress. Encounters with gypsies like Oscar and Johnny provide clues about Isabel's whereabouts and background. The journey transforms Sebastian from a lonely, bullied child into a more confident and compassionate individual, challenging societal prejudices against both himself as an orphan of Romani descent and Belle as a misunderstood animal. His experiences repeatedly highlight themes of prejudice, facing judgment based on appearances.

The culmination involves reuniting with Isabel, proving Belle's innocence to his village, and achieving acceptance for himself and his companion. The resolution sees him maintaining his bond with Belle, solidifying his relationship with Sarah, and integrating his mother into his life permanently. His character arc concludes with a sense of belonging and familial stability, having navigated physical, emotional, and social challenges.