As the book begins, Bella is making her way to the gas station, in her brand new Mercedes Guardian sedan. She is being stared at by one and all, or rather the car is, because it is a gift from Edward and is unavailable in the United States. Apparently, her trust old truck just 'managed' to die on her at the most convenient time. Her thoughts dwell on her impending nuptials, and on her best and currently AWOL friend, Jacob Black. After gassing up the car, she makes her way home to find Charlie being fitted by Alice into his 'monkey suit' and then becomes subject to Alice's machinations with her own final wedding dress fitting. Bella concentrates on her 'happy place' to avoid thinking about anything else.
The night before the nuptials, Edward and Bella are lying in her bed, doing their best make out scene ever. Emmett and Jasper come to spirit Edward away for the Cullen version of a bachelor party. Before leaving, Jasper makes Bella calm down, and smiles as he leaves. But the calming effect doesn't last long, and Bella begins to remember a story that Carlisle has been telling her recently. It is the story of the Immortal Children, an apparent experiment gone awry involving the turning of toddlers and young children into vampires. It is the reason that the vampire world no longer has children in it, an the reason many of them fear the Volturi. The adorable but uncontrollable children and their makers were all slaughtered...including the mother of Tanya and the Denali clan, who secretly created an immortal child of her own. Thinking on this allows her subconscious to run wild, and Bella's nightmares begin.
It is the day of the wedding. Having awoken quite early from her nightmares, Bella's life is temporarily taken over by Alice, who absconds with her to the Cullen household to make Bella up for the wedding. She is polished and perfected and worries most about falling on her face during the ceremony. The wedding itself goes off without a hitch, and the reception begins with no problems. But a surprise comes, in the form of the missing Jacob Black. Edward thanks him, saying it is a wonderful gift for Bella that he shows at all. He passes Bella to Jacob for a dance or two, and all goes well, until they discuss Bella's honeymoon. Jacob becomes furious when Bella spills the beans on what she plans to do with Edward while she's still human. He threatens to kill Edward right then, and begins shaking Bella to knock some sense into her. He is pulled off of her by Edward, Seth and Sam before he can complete his wolfy transformation.
Once things calm down, Edward sweeps Bella off her feet to a place called Isle Esme, a private island off the coast of Brazil, for their honeymoon. They do the nasty, causing Bella all kinds of bruising and Edward copious amounts of guilt. She begins having all sorts of nightmares, eating eggs all the time, and is exhausted constantly. After one particularly horrible nightmare about having a child of her own, she wakes up, frightened, and seduces Edward again. Later on, she tries to cook, throws up several times, and thinks she has food poisoning, until she searches for some Pepto Bismol. She realizes then that she hasn't seen her menstrual cycle in quite some time. Is the impossible true? Is she pregnant? Yes, and Edward wants no part of it, knowing that a vampire baby would kill Bella. He's freaking out, and he and Carlisle make plans for the fetus' quick demise. But Bella has other ideas. She wants the baby, especially after dreaming of it repeatedly, and calls the one person who she knows will defend her decision. After making sure Edward is away for a bit, she makes a call to Rosalie.
So begins the narration from Jacob's point of view. He finds out that Bella is home from her honeymoon, and wants to see for himself if she's been turned into a vampire. Going to the Cullen's home, he finds that she is pregnant, and at an accelerated rate her belly is huge and misshapen. Apparently, a vampire-human hybrid fetus grows to be nine months old within several weeks. He tries to convince her to give it up, per Edward's crazed-man request, but to no avail. Rosalie is now Bella's bodyguard, with no other interest than in Bella's bringing this kid to term; whether or not it kills Bella.
One day, everything is going well, and they all think that they have a few more days before they need to remove the baby. Something goes awry, and while Carlilse is away hunting, Edward, Jacob and Rosalie are forced to remove the baby before all chances of saving Bella are lost. Thus is how Bella becomes a vampire and Jacob's narration comes to an end.
Bella awakes, but the fear of her acting crazed like other newborn vampires is unfounded. She exhibits control that the Cullen's and Jasper have never seen before. She is able to stop herself from hunting and even be in the presence of her own family and Jacob without wanting to kill them. She is able to fight and control her urges.
Bella's baby is a girl, named Renesmee after her mother and Edward's, and manages to have the opposite of her mother's and father's gifts. She is able to show everyone what goes on in her own mind, and no one can block her quite an effective form of communication for a newborn child. However, she is a newborn with the mind of an adult, and within several months, her physical development is to that of a five year-old. Everything is a happy family situation, until they remember the Volturi, who have sent a note saying they cannot wait to see the new Mrs. Cullen. Bella prepares to make a trip to Volterra alone, not wanting the Volturi to know about her daughter. One of the Denali clan foils that idea, however, when she happens upon Jacob, who has imprinted on Renesmee, Bella, and her daughter out hunting in a meadow. Irina sees them, and thinks the unthinkable has happened that Bella's child is one of the Immortal Children. Irina races off to turn the Cullens in to the Volturi.
Alice freaks out one peaceful day, seeing that the Volturi have spoken with Irina, and that they've made the decision that the Cullens all deserve death for the existence of Renesmee. She and Jasper take off, to separate themselves from the rest of the family, saying it's their only hope for survival. But not before the Cullens call all of their friends in existence to bear witness that this is not an Immortal Child, and that if they wish, would they please stick around to let the Volturi know what is going on. All but three are willing, and so they all prepare for the inevitable battle they see forthcoming. This is how they discover Bella's latent talent; that she can shield everyone from the Volturi's powers. As long as Bella projects her power to surround the others, Jane and Alec, the witch twins, cannot attack and subdue anyone within her bubble shield.
The day comes, the Volturi arrive, and they try to make up every excuse in the book to kill the Cullens. They have brought various nomad witnesses of their own, and the Cullens believe they are outnumbered, until the wolves come. There are now more of them, and the Volturi are actually frightened. Even after it is proven the child is not what they think, they still try to kill her and start a fight, basing it on there is no evidence that the child isn't a danger for the future. Bella has to use her talent, secretively, to protect everyone, thereby frightening Aro more in that his typical offensive moves are now null and void. Just as everything looks like the Cullens and their friends might lose, enter Alice, with proof that other children such as Bella's exist. She brings such a child from the Amazon, now an adult, and the Volturi are forced to withdraw.
The final chapter of the book shows everyone getting their happy ending, including Charlie, who has now hooked up with Sue Clearwater, and the other vampires going on about their merry ways, with Jacob forever attached to Bella's child. It stays open-ended, though, leaving room for another book, sometime in the future.