Bella awakens to a new world. Her senses are sharpened, she's beautiful and she's strong - strong enough to make even Edward wince at the enthusiasm of her embrace. Bella wants to see her baby but as a newborn vampire, she is also blood-thirsty and Edward suggests that she hunts and feeds before she sees their baby. During the hunt, Bella is distracted by the unexpected presence of a rock-climber. She rushes to get to the rock climber and Edwards follows, trying to persuade her to go back. Although she can feel the thirst for blood burning in her lungs, Bella's will power and self-control prove so strong that she is able to break off in mid-pursuit of the rock-climber to follow a small deer. She sees a mountain lion with the intent to feed on the deer as well, so she attacks and feeds off the mountain lion.
Well-fed, she returns to the Cullen home, anxious to see her baby. Jacob meets her outside and expresses concern that Bella isn't ready, much to Bella's annoyance. Jacob insists that Bella get used to his smell for the well being of the baby. Bella is confused by this and Edward shakes his head at Jacob, warning him to not tell Bella that he has imprinted on Renesmee. Bella realizes now that Jacob, being a wolf, really does smell bad to her. Edward assures Jacob that Bella can handle meeting Renesmee and Bella is at last reunited with her. Renesmee puts her hand on Bella's cheek and shows her their first memory. Jacob is still fretting and the entire clan watches with varying degrees of amusement as Bella realizes that Jacob has "imprinted" on Renesmee and is thus bonded with her for life. Angrily, she confronts Jacob, who tries to reason with her. She shoves him and tells him that he doesn't have any rights to her child. Seth and Leah, in their wolf forms, try to defend Jacob and Bella angrily knocks Seth into a tree when he moves to protect Jake. She is instantly contrite. Jake is able to then persuade her that he and Renesmee are good for each other and she accepts he has no choice in whom he imprints. He explains that it doesn't mean what she thinks it means and that he only wants to protect Renesmee. He says that it all makes sense now, why Jacob and Bella could've never been together, why Bella wanted to be around Jacob so much during her pregnancy... it was Renesmee who wanted him around the entire time.
As a wedding gift, Esme has renovated a cottage on the property for Bella and Edward to give them some privacy. They again make love, which Bella enjoys even more as a vampire. Bella wonders why they would ever want to stop making love since it feels so great, they never have to stop to rest or sleep. They return to the Cullen's and the phone is ringing. It's Charlie; he has been calling twice a day. She misses Charlie, and is saddened when Carlisle points out that the kindest thing to do would be to tell Charlie that she has died and for them to move on. Jacob is taken aback by the prospect of losing Renesmee, and argues that they cannot leave Forks. The Cullens see no other way around the problem. Jacob takes the matter into his own hands by exposing himself as a werewolf to Charlie - Charlie is embarrassed and alarmed as Jacob awkwardly as begins to strip with a fumbling explanation, and shocked when he then turns into a wolf. Jacob explains that Bella has also undergone an "unusual" change but wants to have him in her life, if he can handle it. Edward is angry when Jacob tells them what he has done, reminding Jacob that Bella is still new to being a vampire, and that Jacob's actions may have endangered Charlie. With hurried coaching from her new family, taught to fake blink, breathe and move slowly. Bella manages to control her blood thirst when Charlie visits and act almost normal, telling him only what he needs to know, even though he realizes that she has changed. He is hurt and confused since his daughter looks like his daughter but doesn't. When they embrace, he notices her cold body. They introduce Renesmee as a niece that they have adopted, but Charlie isn't a foot -- he realizes that she has Bella's eyes. He agrees that he does not need to know and falls in love with his "special" grandchild. After Charlie leaves, Emmett jokingly remarks that Bella seems too tame to be a true newborn. But Bella proves she has the physical strength of a newborn by besting Emmett in arm wrestling, and then smashing a huge boulder as the other Cullens watch in amusement.
In a matter of weeks, Renesmee can talk and grows big enough to walk and hunt, which is a matter of concern for the family, because they fear she will age too quickly and die. When Bella and Jacob take her on a hunting trip, Irina, a sister of the Denali clan, sees Renesmee from a distance, and mistakes her for an "immortal child," a child that has been made a vampire -- one of the highest crimes under Volturi law, because immortal children are undisciplined, never become mature, and can slaughter whole villages in one outburst of emotion. This would make it impossible to hide the existence of vampires from humankind. Bella is troubled that Irina refuses to speak to them and flees at Bella's approach, but Bella shrugs it off after Edward and Carlisle theorize that Irina was offended at the sight of Jacob in his wolf form. But later, Alice foresees the Volturi marching to destroy Renesmee and the whole Cullen clan. When she notes that Irina was with the Volturi, Carlisle puts everything together and realizes that Irina reported the Cullens to the Volturi.
Shortly after, Alice disappears with Jasper, leaving only a brief note of instructions written on a page torn from Bella's copy of 'The Merchant of Venice'. The note suggests that they gather witnesses to vouch for Renesmee, because the Volturi will appear when the snowfall sticks. Alice and Jasper's disappearance frightens the family, who is afraid that Alice left because she foresaw their doom. Curiously, Bella refers to her book to further find a cryptic note leading her, and only her, to a man named J. Jenks, in Seattle. Bella makes an appointment with him and meets him at a restaurant -- private, yet public; Mr. Jenks is sufficiently wary of the Cullens to protect himself. She discovers that Jasper has ordered a fake passport for Jake and Renesmee, as well as a driving license and birth certificates. Bella realizes, with dismay, that Alice has foreseen her and Edward's death and this is Alice's way of protecting Renesmee.
To stop the Volturi, the Cullens decide to call in their friends from around the world to witness that Renesmee is not an immortal child, but is half-human with a heart beat. Friends come from all over -- Russia, Egypt, India, South America. They are wary at facing the Volturi, and many of them have their own axe to grind with the Volturi's rule. The Denali "cousins" explain that Irina was particularly appalled by what she saw as an immortal child, because her mother was executed by the Volturi for creating one. Immortal children were beautiful and everyone loved them, becoming extremely attached. But the children could not learn the rules and would massacre entire villages in one night, causing panic and rumors and threats to all Vampires. The Cullen's friends who agree to come all have their own special gifts. Benjamin can control the elements. Zafrina and Senna can make others see whatever they want them to see. Making Edward believe he was in the middle of a rain forest.
More and more members of Jacob's tribe start to turn into wolves due to the high number of vampires entering the area. The pack works to train and explain everything to the new members. The wolves also promise their support to fight the Volturi if need be. Training for a possible fight against the vast numbers of the Volturi army, Bella discovers that her special power is "shielding", which is the reason why Edward, Aro or Jane weren't able to read her mind or cause her pain. She practices projecting her shield to protect others from the mental powers of even the fiercest Volturi. While she is practicing with Kate, Kate uses her electric shock on Edward to motivate Bella to extend her power on to another person, in order to protect the ones she loves.
After the first snowfall, the Cullens and their friends gather in a large field, awaiting the Volturi, who advance majestically en masse. Carlisle tries to reason with Aro. Aro calls Edward forward and reads his thoughts to discover that Renesmee is not an immortal child, but is half vampire and half human. He calls Renesmee forward. Bella, Jacob and Emmett come forward with her. Aro is charmed as she shows him her history and nature. Aro realizes Irina was mistaken. Irina admits that the child she saw was smaller and apologizes for her mistake, taking full responsibility. Aro moves to his plan B, to provoke the group with the execution of Irina. Tanya and Kate, distraught at the sudden and shocking death of their sister, immediately respond, and it takes the work of the rest of the allies to control them, Zafrina and Senna temporarily blind them to calm and slow them. They realize it was his plan to invoke a violent response. As the Cullens and their supporters regain their control, Aro moves on to Plan C and suggests to his followers that because Renesmee is unprecedented, she is unpredictable and therefore unsafe.
Suddenly, from the woods, Alice and Jasper stride forward into the clearing, to show Aro she has seen the future and Renesmee poses no threat. However, in a blink, suddenly, Alice gasps in consternation as she realizes that no matter what she shows Aro, he has already decided on the future of the Cullens: they must be destroyed because they are a threat to his rule and he wants the talented individuals under his control. Alice turns and gives Bella a surreptitious signal that it is time for Jake to take Renesmee to safety. Alice then turns and kicks Aro across the field. Grimly, Bella sends Jacob and Renesmee away, and as Jacob speeds away, she turns to face the Volturi. Jane attempts to weaken Edward with her power and is frustrated when she realizes that Bella, with a satisfied smile, is protecting him with her shield. Alice and Jasper are captured by the Volturi, which brings Carlisle flashing forward in loud protest. Shockingly, Aro defeats Carlisle in one blow, decapitating and burning his body. The Cullens and their allies are enraged at Carlisle's death, and a violent battle ensues between the allies and the Volturi, where there are many fatalities on each side, including Carlisle, Jasper, Seth, Leah, Alec, Jane, Demitri, Aro, Caius and Marcus. Jacob and Renesmee are pursued but Jacob decapitates the vampire after shortly being subdued due to the death of Leah. Benjamin smashes his fist into the ground, causing the earth to crack open, revealing lava far below. Edward barely escapes being sucked down into the crevice. Esme claws desperately to keep from falling in as a Volturi clings to her back trying to kill her. Seth saves Esme, destroying the Volturi attacking her, causing himself to fall instead. Alice, enraged over Jasper's death, attacks Jane while protected by Bella. Alice catches Jane and throws her to Sam, who gladly decapitates her. Caius fights valiantly against the Denali until Kate's electric shocks wear him down. Marcus is relieved as enemies close in on him and welcomes his death. Finally, Bella and Edward see Aro and go for him. He knocks them both over. Edward grabs Bella and throws her into a kick that knocks over Aro. They then grab him by the arms and head, killing him. Chaos reigns . . .
Suddenly, the scene shifts, and we return to Alice gazing at Aro: She has shown him (and us) the future she has foreseen if he continues on this path, and has made him realize that they will die if their meeting turns into a war. She tells him that now he knows what will happen and should just leave now. Aro tries to back off, but the others are still worried that since they know nothing of Renesmee, she could still be dangerous. Alice waves forward a young Brazilian named Nahuel, another vampire-human crossbreed Alice has found in Brazil. He approaches with his aunt and testifies as to his background and genetic makeup to prove that he is no threat. A vampire seduced his mother and she dies in childbirth, his aunt raised him and he turned her. Bella is relieved to hear that during his aging process, he became fully grown after 7 years, but has been alive (and no threat) for the last 150 years. He can survive on blood or regular food. Faced with this evidence as well as Bella's powerful shield, the Volturi gather what shreds of dignity they have remaining and take their leave. The Cullen's allies also begin to depart, with the Cullen's grateful thanks. Bella, Edward, Jake and Renesmee return to their home in peace to live happily ever after. Alice, smiling, sees a future Bella and Edward warmly greeting and embracing a laughing, happy adult Renesmee and Jake on a beach. Edward catches Alice's thoughts and smiles with relief at the knowledge that they have succeeded in making life safe and happy for their loved ones. Jacob catches the serene expression on Edward's face as he watches Bella with Renesmee, and mischievously wonders aloud if he should start calling Edward "Dad". "No," says Edward firmly, but a small smile plays across his face at the irony.
In the last scene, Bella and Edward are sitting in the meadow we have come to know from the previous films, and Bella surprises Edward by lifting her shield to allow him to read her mind, showing him her memories of their life together so far. They kiss, as a phrase from the end of the book reminds us that they will be happy forever.