The movie takes place in the 10th century Scotland. Princess Merida is an incredibly skilled archer living in the Scottish kingdom of DunBroch with her mother Queen Elinor, and her father King Fergus. On Merida's sixth birthday, King Fergus gives Merida a short bow and arrow as a birthday present. After retrieving an arrow accidentally fired into the woods, where Merida sees some Will o' the Wisps, Merida's family is attacked by an ancient bear, Mor'du. Elinor and Merida flee, while Fergus and his men stay behind to battle the bear. The story cuts to around ten years later when we learn that Fergus lost his left leg during the battle with Mor'du and has vowed to find Mor'du again and avenge his leg.
Ten years later, despite Elinor's intention of having sixteen-year-old Merida become a proper, royal, ladylike princess, Merida is determined to seek freedom and live her own life. One day, the Lords Macintosh, MacGuffin and Dingwall arrive to present their sons for Merida's betrothal. After an argument, all three armies get into a brawl, which Elinor stops. She states that the challenge which the first-born of each clan would compete in would be decided by the fair maiden. Merida, hearing this, believes that the idea of the contestants being first-born is a loophole, so Merida chooses archery. The first two contestants lack skill, and Lord Dingwall's son wins by a fortunate mishap. Merida then steps forward with her bow and claims that she represents her clan as first-born and will be shooting to claim herself. Despite Elinor's objections, she shoots all three bull's-eyes, even splitting Dingwall's son's arrow, supposedly winning the contest.
Merida and Elinor have a falling out, with Elinor claiming Merida will still have to choose a suitor. Merida slashes the family tapestry in anger and Elinor throws Merida's bow into the fireplace. Merida runs away on her horse in tears as Elinor quickly retrieves the bow from the fire in regret. Merida again comes across Will O' the Wisps, small blue fairies that are said to lead you to your fate. Merida follows a trail of them to an old hut, where she finds a witch who claims she is nothing but a humble woodcarver because her last customers had some objections to her spells. The witch says she won't help Merida unless she buys a wood carving; in response, Merida buys the lot, trading for a necklace, but says she wants a spell that will change her fate as well. Merida asks for a spell that would change her mother, which she assumes will change her fate. The witch conjures a tart for Merida to give to her mother. On Merida's way out, the witch thinks she forgot to tell Merida something about the spell, but she and her cottage disappear when Merida turns around to ask what the witch was saying.
Back in DunBroch, Fergus is entertaining the lords, while Merida finds Elinor who was worried. After a brief conversation, Merida presents the tart to her mother, who takes a bite and immediately feels sick. Merida takes Elinor up to her room, where she rolls off her bed and Merida discovers that her mother has transformed into a bear. Fergus, downstairs in the hall, smells a bear, and he leads the lords upstairs. Merida and Elinor, with the help of her the young princes Harris, Hubert and Hamish, manage to escape the castle without Fergus finding them, and she and her mother retreat into the forest back to the witches cottage. The witch isn't present, but there is a cauldron where the witch appears and has left a cryptic message for Merida. She tells Merida that by the second sunrise the spell would be permanent unless Merida can repair a bond. Merida and her mother exit the cottage and find shelter for the night, wondering how they'll be able to fix the curse.
In the morning, Elinor has set out a breakfast of some found berries, which turn out to be poisonous, and some dirty water. Merida finds a river and takes the time to teach her mother how to catch fish. The two bond, seeing different sides of each other. While in the forest, Elinor suddenly acts as if she's about to attack Merida, but soon recovers as Merida remarks that her mother is turning into a true bear. They then come across some wisps and are lead to the ruins of an old castle. Merida falls down a pit into an old throne room which she realizes is the palace of the four brothers from Elinor's story. She finds an old stone engraved with the four brothers, but finds a part with a brother had been cut off, as when Merida had cut the tapestry of her own family earlier. She realizes that the curse has happened before and that the brother who'd gone on his own path is now Mor'du. Then Merida is attacked by Mor'du who lives in the ruins, but with the help of her mother she escapes.
Merida realizes Elinor will become just like Mor'du, like a real bear, if they don't break the spell. They travel back to DunBroch to sew the family tapestry back together, thinking that will break the spell. To get Elinor upstairs without bein