In Chapters 1 through 12, Mary Lennox learns to be a decent person with the help of a lovable country boy named Dickon Sowerby and about a kajillion climbing roses.
When you start reading The Secret Garden, you may have a tough time believing that ten year-old Mary Lennox is the heroine of the novel: She's sour, mean to her servants, and totally self-absorbed. But we spend the first act of the novel coming to see that Mary isn't so bad after all. It's just that she has spent most of her early life being bored and lonely.
Once she moves into her uncle's giant and mostly empty house in rural Yorkshire, she finds something to do (taking care of the Secret Garden) and someone to talk to (Martha and Dickon Sowerby and Ben Weatherstaff). As Mary learns to think about things other than herself, she becomes a much more likable person.
In Chapter 13, we add a new spoiled brat added to the mix: Colin Craven. We really see how much Mary has improved when we can compare her to her self-centered, bullying cousin.
The fact that it takes Mary eleven chapters to find a kid who lives in the same house with her proves two things: (1) Colin Craven is really isolated from the world around him, and (2) Misselthwaite Manor must be huge. (Side note: If you think it's strange that Mary only finds Colin after she's been living in Misselthwaite Manor for several months, check this out.)
Since Colin's mother passed away when he was born and his nurses basically give him anything he wants so that he'll stay calm and quiet, he has no one to talk to who treats him like an equal. Mary becomes something like a mother and a sister to Colin, singing lullabies to him but also fighting with him when he is being an unreasonable brat.
With the help of Mary's tough love, Dickon's kindness, and the Secret Garden's natural beauty, Colin's negative patterns of thinking about himself and other people start to change. He becomes a much nicer guy when he stops thinking only about himself and his own worries. Go figure.
Mary and Colin have both grown into happy, active kids. When Archibald Craven comes home and reunites with his son, they form a happy family at last.
As Colin slowly teaches himself to walk, he decides he doesn't want to tell his father that he's in the middle of healing himself physically. He wants to surprise Archibald with his walking in the hopes that his new strength will make his father happy at last. And, amazingly, this secrecy works: In Chapter 27, Archibald returns to Misselthwaite Manor. He's worried that Colin may actually have gotten more bratty and evil now that he's getting older. But when he goes out to the Secret Garden, Archibald finds Colin totally transformed into an active, healthy kid.
Colin shows Archibald around the Secret Garden and the two of them start to build a new family relationship.