Breaking Dawn is divided into three separate parts. The first part details Bella's marriage and honeymoon with Edward, which they spend on Isle Esme, a private island that Carlisle bought for Esme, off the coast of Brazil. Two weeks into their honeymoon, Bella realizes that she is pregnant with a half-vampire, half-human child and that her condition is progressing at an unnaturally accelerated rate. After contacting Carlisle, who confirms her pregnancy, she and Edward immediately return home to Forks, Washington, where the fetus continues to develop at an abnormal speed.
The second part of the book is written from the perspective of Jacob Black, a werewolf who had also fallen in love with Bella. A month after the wedding, Bella calls her father, Charlie Swan, and says that she is sick. Charlie relays this to Billy Black, Jacob's father, and Billy relays it to the pack. Jacob thinks that Bella is now a vampire and attempts to lead an assault on the Cullens for breaking the treaty. When he arrives, he discovers that Bella is sick, but it is from her pregnancy, not a disease. Jacob begs Bella to abort the fetus and save her life, but Bella is determined that she will live long enough to have the baby and be transformed into a vampire. Jacob runs off in his werewolf form and keeps thinking of what Bella had said. This accidentally reveals Bella's condition to the pack, and because they think the baby will become a monster, the pack decides to kill the baby and Bella immediately. Jacob refuses, and when Sam uses an alpha command to force Jacob, Jacob accepts his bloodline as the true leader of the pack and breaks free of Sam's control. He runs off to warn the Cullens of the pack's plan to kill Bella. However, while en route, Seth Clearwater, one of the younger pack members, joins Jacob and makes note of the fact that neither of them can hear the others' thoughts and that they had formed their own pack. They warn the Cullens and take up patrol around the Cullens' property. Leah, Seth's older sister, joins them shortly afterward. After a chance remark by Jacob, they discover that the baby simply craves blood, and Bella has to drink it out of a cup for the greatest effectiveness. Bella gets better, but the fetus grows swiftly and the timing of birth is unknown due to the lack of any related medical knowledge on human-vampire births. Carlisle, the experienced doctor, is out of the house to replenish blood supplies from a hospital blood bank when Bella falls, detaching the placenta and inducing labor. The baby breaks many of her bones, including her spine, and she loses massive amounts of blood. In order to save her life, Edward changes her into a vampire by injecting his venom into her heart. Jacob, thinking that Bella is dead, and blaming Bella's daughter Renesmee as the cause, tries to kill Renesmee. Instead, he "imprints"—an involuntary response in which a shape-shifter finds his soul mate—on her.
The third section shifts back to Bella's perspective, describing Bella's painful transformation, waking to find herself changed into a vampire and enjoying her new life and abilities. However, the vampire Irina misidentifies Renesmee as an "immortal child," a human child who has been turned into a vampire. Because immortal children are uncontrollable, creating them has been outlawed by the Volturi. After Irina presents her allegations to the Volturi, they plan to destroy Renesmee and the Cullens. In an attempt to survive, the Cullens gather other vampire clans from around the world to stand as witnesses and prove to the Volturi that Renesmee is not an immortal child. Edward telepathically connects with Jacob and his Quileute pack, with the full realization of Jacob's imprint on Renesmee forging a new family connection of an unbreakable bond and mutual pact of protection between the Cullens and the Quileute, ending hatred between the races. As local and foreign vampires arrive, the Cullen house becomes the headquarters and training ground for the assorted vampires and the Quileute wolf packs. Bella's human ability shielded her from psychic powers, but as a vampire she learns she can project it to shield others in a wide radius, a secret weapon that neutralizes the Volturi's powers. Upon confronting the gathered Cullen allies and witnesses, the Volturi discover that they have been misinformed on Renesmee's identity, and execute Irina when she refuses to make a claim against the wolves for killing Laurent, trying unsuccessfully to instigate a full battle. Additionally, Caius brings up the Cullens' pact with the Quileute as allying with the vampires' sworn enemy, the Children of the Moon, but it turns out the Quileutes are "shapeshifters" that chose the form of giant wolves, and not Lycans. The Volturi remain undecided on whether Renesmee should be viewed as a threat to vampires' secret existence. At that time, Alice and Jasper, who had left prior to the gathering of the Cullen's allies, return with a Mapuche called Nahuel, a 150-year-old vampire-human hybrid like Renesmee, and his biological aunt, Huilen. Huilen tells of how her sister fell in love with a vampire and became pregnant with his child. She also shares that when Nahuel was born, he bit her and made her immortal. Nahuel demonstrates that the hybrids pose no threat, and the Volturi agree to leave the Cullens alone. Before the Volturi leave, he also informs them of his father's intent to create more hybrids to produce a "super race." While allowing them to deal with his father, Nahuel begs the Volturi to spare his sisters. The Cullens and their allies return to their homes in peace, accepting that the Volturi may one day return. When Edward and Bella are alone, she lets him read her mind for the first time, sharing her feelings about him from her memories. He asks to see her memories again, but she tells him it would take time to show him again. Edward reminds her not to worry, as they have eternity to spend together.