When her parents die, spoiled Mary Lennox (Kate Maberly) is sent from India to England to live with her uncle Archibald Craven, a mysterious and lonely man. He's rarely home, leaving Mary to the care of housekeeper Mrs. Medlock ( Maggie Smith ). One day, Mary finds the key to THE SECRET GARDEN, once the favorite place for her uncle and his wife, whom he adored. After she died, he locked it up and swore no one would go in there again. Mary decides to find the garden, but first she investigates the crying she often hears in the night and discovers her uncle's son, Colin (Heydon Prowse). He has been confined to bed all his life and is spoiled to the point of hysteria. Mary soothes him by telling him about the garden. Later, when he has a tantrum, she is the first person ever to set limits on his behavior. Mary finds the garden, and she and Colin and the maid's brother Dickon bring it back to life. As they do, Mary and Colin get stronger in body and in spirit. When Archibald returns, he meets them in the garden. They run to him, and it is clear that the garden will heal him, too.
Colin, Mary, and Dickon spend all of their time in the garden, where Colin learns to both stand and walk on his own. Anxious to show Colin's new-found life to his father, they perform a "magic" ceremony in hopes to bring him back home. It appears to work, as Lord Craven awakens suddenly from a dream of his late wife calling him home. He immediately returns to Yorkshire. He discovers Colin walking and playing upon his return, which leaves him dumbfounded with joy. Mary runs off and breaks down in tears, fearful that both she and the garden will be neglected and locked away again. Her uncle reassures her that she is part of the family and promises to never lock the garden up again. He thanks her for bringing his family back to life; they embrace, and then celebrate with Colin, Dickon, and the Manor staff. The film concludes with Mary reflecting in voiceover that "if you look the right way, the whole world is a garden."