For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
We spend the early parts of the book learning about Margaret Hale's proud personality and her love for her home village of Helstone, which basically looks like a big garden. Her life gets turned upside down, though, when her father quits his job as a pastor and moves their family to the dirty northern factory town of Milton. Margaret has a very tough time adjusting to the business-minded town, where money seems to be the measure of all value.
Act II
Not long after moving to Milton, Margaret's mother falls ill. The doctor says she'll die, so Margaret writes to her brother Frederick, who is living in exile in Spain after heading a mutiny in the English Navy. Frederick returns to England at great risk to see his mother before she dies. After her death, Frederick makes a narrow escape at the Milton train station and eventually gets out of the country safely.
Meanwhile, a Milton businessman named John Thornton has fallen in love with Margaret and asks her to marry him. She refuses, though, thinking that he's not good enough for her because he doesn't aspire to anything in life other than money. He's also not really a supporter of the rights of his workers, which peeves Margaret.
Months later, Margaret's father also dies, leaving Margaret alone.
Act III
After her father's death, Margaret moves to London to live with her cousin's family. Following her father's death, a family friend named Mr. Bell has decided to look out for Margaret's best interests. To this end, he makes her the sole beneficiary of his will.
When he dies of a sudden stroke, Margaret finds herself very wealthy. She learns that since she left Milton, John Thornton has become a kind and compassionate boss. The problem is that he has also lost a lot of money and it doesn't seem like his factory will stay afloat. Oops.
When he visits London to talk about subletting his house, Margaret offers to loan him the money he needs to keep his factory going. Taking this as a profession of love, Thornton asks Margaret again if she'll marry him. This time, she says yes. D'awwww.