In 1901, recently orphaned 10-year-old Mary Lennox is sent from her home in British India to her uncle Lord Archibald Craven's mansion, Misselthwaite Manor, in Yorkshire, England. Unloved and neglected by her late parents, who were killed in a sudden earthquake, she is a cold and self-centered girl who has repressed her emotions to the point of being unable to cry.
Mary is unhappy in her new surroundings. Head housekeeper Mrs Medlock informs Mary she will not be spoiled as she was in India and that her uncle, who spends extended periods of time away from the Manor, will likely not see her. Mary is ordered by Mrs Medlock not to leave her room, but strange noises lead her to explore the mansion on her own; until at one moment, her curiosity leads her to get caught by Medlock who locks Mary up in her room as punishment and warns her about the consequences if she does it again. Eventually Mrs Medlock allows her to play outside to keep her from poking about in the house. In the expansive grounds of the Manor, Mary discovers her late aunt Lillian Craven's walled garden, which has been locked up and neglected since her accidental death 10 years prior.
Martha Sowerby, a cheerful housemaid and her younger brother Dickon Sowerby, a nature-loving boy who can "talk" to the animals, befriends Mary. Fascinated by the secret garden, Mary enlists Dickon to help her bring it back to life, gradually becoming a more friendly and happier child in the process. When she is finally introduced to her uncle, Mary is apprehensive, knowing he was responsible for locking up the secret garden. Fearful he will do it again, she evasively asks to plant seeds in an "unwanted" part of the Manor. Lord Craven grants permission and leaves the country for the rest of the year. Confident that the garden will remain a secret, Mary and Dickon continue their work.
Hidden away in the gloomy mansion is Mary's cousin, Colin Craven, who has been treated like a fragile, sickly invalid his entire life. This has turned him into a spoiled, demanding, short-tempered boy (similar to how Mary was back in India) who has never left his room or even learned to walk. Mary eventually discovers Colin and learns the strange noises she has been hearing are the sounds of his crying. She is taken aback by his difficult nature, but reaches out to him anyway. She shows him that he's not really sick, and that the outside world is not as dangerous as Mrs Medlock, who is in charge of caring for him, claims. Encouraged by Mary, Colin decides to go outside for the first time in his life. Mary and Dickon take him to the secret garden, and Colin begins his own healing process, both physically and mentally.
Mary, Colin, and Dickon spend all of their time having fun in the garden, and Colin learns to stand and walk on his own. Anxious to show Colin's new-found life to his father, they perform a magic ceremony, hoping to bring him back home. It appears to work: Lord Craven awakens suddenly from a dream of Lillian calling him home and immediately returns to Yorkshire. He finds his way into the secret garden, where he discovers Colin walking and playing Blind man's buff with Mary and Dickon, which leaves him dumbfounded with joy.
Upon seeing her uncle, Mary runs off and breaks down in tears for the first time in her life, certain that nobody wants her and afraid that the garden will be locked up again. Lord Craven catches up to her and reassures Mary that she is part of the family now. He promises never to lock the garden up again. Lord Craven thanks Mary for bringing his family back to life. Mary, Colin, Dickon, and Lord Craven embrace, then celebrate with Martha, Mrs. Medlock, Ben, and the Manor staff.
The film ends as Dickon is shown riding his horse in a meadow while a voiceover of Mary reflects that "If you look the right way, the whole world is a garden."
Kate Maberly as Mary Lennox
Heydon Prowse as Colin Craven, Lord Craven's son and Mary's cousin
Andrew Knott as Dickon Sowerby, Martha's younger brother
Laura Crossley as Martha Sowerby, Mrs. Medlock's servant and Dickon's older sister
John Lynch as Lord Archibald Craven
Maggie Smith as Mrs. Medlock, Lord Craven's servant and housekeeper
Irène Jacob as Mrs. Lennox/Lilias Craven
Peter Moreton as Will
Colin Bruce as Major Lennox
Walter Sparrow as Ben Weatherstaff, the gardener
based on the 1911 novel of the same name by Frances Hodgson Burnett. The novel was previously adapted into two films: a 1949 drama film and a 1919 silent film, which starred Lila Lee and Spottiswoode Aitken.