Honestly, The Secret Garden has such a straightforward plot that we can almost sum it up as follows: Girl loses parents, girl finds friends, girl finds garden, boy joins girl in garden, boy learns to walk on his own, the end. Actually, that isn't quite as straightforward as we thought; let us explain in a bit more detail.
Mary Lennox is a nine-year-old British girl growing up in colonial India in the care of a sequence of nannies. Since her father is an officer in the British army and her mom is super-busy with the vital business of dinner parties and nice clothes, Mary barely knows her own parents. And since she spends all of her time alone, she's selfish, demanding, and self-absorbed. Her parents die suddenly of cholera, leaving her in the care of her mother's brother, Archibald Craven.
Mary's uncle doesn't care much about her, so he brings her to his huge mansion in England, Misselthwaite Manor, and basically leaves her there, more or less on her own. Mary's maid Martha is a cheerful Yorkshire woman who won't stand for Mary's spoiled tantrums and fits. She tells Mary all about two things that change Mary's life: (1) There is a walled garden on the grounds that has been sealed off since the death of Archibald Craven's wife ten years before; and (2) Martha has a little brother named Dickon who loves gardening and wild things.
Of course, Mary discovers the walled garden (since the title of this book is The Secret Garden). With the help of a local robin (this isn't Mary Poppins, so the robin doesn't actually talk, but it's smarter than your average bird), Mary stumbles on the long-lost key to the garden and opens it up.
With the kindly help of nature-smart Dickon, Mary begins secretly working in the garden to bring its many roses back to life. The exercise and outdoor time improves both her physical and her mental health, and Mary stops being quite so much the spoiled princess that she was at the beginning of the novel.
Ever since she first arrived at Misselthwaite Manor, Mary has been hearing the sound of a crying child late at night. She finally discovers the secret of the Manor (well, besides the Secret Garden) one night: Archibald Craven has a son. The boy, Colin, is even more sheltered and spoiled than Mary. He's been told his whole life that he is sickly, so he believes it—even though there is actually nothing physically wrong with him. Since he gets so little exercise and he spends so much time about his own (imaginary) illnesses, he has a rotten temper and horrible manners.
Now that Mary is around to give Colin some straight talk about his bullying behavior and his needless self-pity, though, he begins to grow out of his selfishness. The two of them (again, with the help of Dickon and his green thumb) decide to work in the Secret Garden together. Colin decides that he is going to make himself better so that when his father arrives back in England, Colin can surprise Archibald with his transformation.
But wait… You may ask: Why does Colin's father spend so little time at home with Colin? There are two important details to this tragedy that you have to know: (1) Archibald has a deformed spine, which has had a huge emotional effect on him; and (2) he was deeply in love with his wife Lilias. Lilias was pregnant with Colin when she fell from a tree in the walled garden, went into labor, and died. That's when the walled garden becomes the Secret Garden: Archibald ordered it sealed up, since Lilias was horribly injured there.
Colin survived this terrible start in life, but Archibald can't stand to look at him. Archibald hates that Colin is all he has left of Lilias, and he also worries that Colin is going to deal with the same physical difficulties with which Archibald struggles. So Archibald has spent most of Colin's life traveling in Europe and leaving Colin in the care of his doctor and housekeeper. It's only with the help of Mary and Dickon that Colin begins to imagine that he might survive, and even thrive, despite his mother's untimely death.
As Colin begins to think less and less about himself and more and more about the Secret Garden and the natural world around him, he realizes that he is not going to die. Colin resolves to spend his life exploring the wonders of Nature. With the strength he gets from this new appreciation for life, Colin slowly learns to walk on his own two feet.
Meanwhile, as Colin's health (and behavior) improves, Archibald has a strange dream in which his deceased wife tells him to go back to the garden. Archibald rushes back home to Misselthwaite Manor at once, only to find that his son has grown into a fine, sturdy young man whom Archibald barely recognizes. As Colin shows his father around the reborn Secret Garden, father and son begin to build a relationship they've never been able to have before.