Matthew and Marilla Cuthbert, an elderly unmarried brother and sister, run the Green Gables farm in the quiet Prince Edward Island village of Avonlea. Wanting help with the work, they arrange to adopt a boy from an orphan asylum. Through a muddle, the child who waits at the little station is a girl: thin, freckled, red-haired Anne Shirley, about eleven, who talks without pause and imagines without rest. Shy Matthew cannot bring himself to send her back and carries her home to let Marilla settle the matter.
Marilla means to return the mistake, but Anne's history of being unwanted, and her fierce, funny way of meeting the world, work on her. The Cuthberts decide to keep her. What follows is a string of episodes in which Anne's vivid temper and high-flown fancy collide with Avonlea's plain customs: she names the pond the Lake of Shining Waters, swears eternal friendship with Diana Barry as a kindred spirit, and falls into one mortifying scrape after another, from dyeing her hair green to accidentally getting Diana drunk on what she thought was cordial.
Her pride shows its hardest edge at school. When the handsome Gilbert Blythe teases her about her red hair by calling her Carrots, Anne brings her slate down on his head and refuses, for years, to forgive or even speak to him. The grudge runs alongside a fierce rivalry to lead the class, and that rivalry quietly sharpens her into a serious student under the encouragement of her teacher, Miss Stacy.
As Anne grows, the comedy deepens into something steadier. She studies for the entrance examination to Queen's Academy, wins a place, and earns honors there along with a scholarship that would carry her on to college. Throughout, the reader watches not only Anne change but the household around her soften, as the once-crisp Marilla grows mellow and the bond between the three of them becomes the real center of the book.
Then Matthew dies suddenly of a heart attack, and the loss reorders everything. Marilla's eyesight is failing, and selling Green Gables looms. Anne gives up her scholarship to stay, teach nearby, and keep the home she has come to love. The story closes with her reconciling with Gilbert and looking down a future she pictures as a bend in the road, narrower than she once dreamed but still bright with quiet happiness and the promise of what lies unseen around the turn.