Isabella Swan, (more commonly known as Bella Swan), decides to move from sunny Phoenix, Arizona to rainy Forks, Washington to live with her father, Charlie Swan, whom she has only seen during summer vacations due to her parents' divorce. Bella does this to allow her mother to travel with her new husband, Phil Dwyer, a minor league baseball player. Never having had many friends before, at Bella's new school she is quickly accepted, and finds she has several boys competing for her attention.
Over time, Bella begins to find Forks boring and monotonous. The exception is Edward Cullen, the mysterious boy she sits next to in her biology class. The enigmatic Cullen family, all of whom are described as being inhumanly beautiful, have the reputation of being very reserved and clannish. From her first experience with Edward, when he flashes her a look of hate, to his attempts to be moved from Bella's class, she finds herself unable to stop thinking of him. Through careful observation, Bella realizes that Edward is not quite normal, but despite having many theories cannot bring herself to believe that there is a supernatural explanation for Edward's behavior.
Bella believes Edward hates her, but this begins to change. He displays superhuman abilities in saving Bella from being hit by an out-of-control van about to crush her in the student parking lot of the high school. She confronts him with the strangeness of the situation but he refuses to talk to her about it. As time passes, Bella and Edward are time and again drawn to each other despite Edward's best efforts to keep his distance from her.
Bella eventually learns, from her friend Jacob Black of the Quileute (pronounced Quil-yoot) tribe, and her own observations, that Edward and his family are reformed vampires who substitute animal blood for human blood in their diet. Because of their diet, the Cullens have tawny- or honey-colored (described by Bella as being topaz) eyes that will slowly change to black (onyx) as they become hungrier or thirstier. Those vampires who drink from humans have red irises. The way to tell the length of time between blood feedings is the amount of color around their pupil. The darker a vampire's eyes are, the longer it has been since they have hunted.
The members of the family include Carlisle, Esme, Rosalie, Emmett, Jasper, and Alice. Carlisle is the oldest and the head of their family. He has become used to the smell of blood in order to do what he loves: being a doctor to humans. His wife, Esme, has a very sweet, caring disposition. She is the mother to the entire family. Esme once revealed to Bella that she has lost a child of her own, thus Edward's happiness means a great deal to her, because she loves him just as much as she would love her own son. Rosalie is Emmett's mate,although she was originally intended to be Edward's. Emmett is very large and muscular with a sunny disposition and bearlike countenance. Alice is clairvoyant and ends up befriending Bella. Her clairvoyance often comes in the form of visions. Alice also has a problem that none of the other Cullens do, in that she cannot remember her life before she was changed. Jasper is her mate and has the most trouble adhering to the Cullen's diet, since he is the newest member of the family. He has the ability to manipulate the emotions of those around him by emanating a calming influence. Edward's gift is that he can hear what anyone around him is thinking, with the single exception of Bella herself. We later find out that Bella is also immune to every other psychological "gifts".
Despite the obstacles presented by the Cullen's vampiric nature, Edward and Bella fall in love. Their foremost problem is Bella's alluring scent, which makes it difficult for Edward to restrain himself from drinking her blood. However, despite this, and Bella's strange knack for constantly landing herself in dangerous situations, they manage to stay together, safely, for a time. Bella's lack of fear regarding Edward's nature as a vampire, her constant longing for him, and the fact that she is impervious to his gift for reading minds throws Edward off balance. He returns her love but is always confused as to why Bella feels the way she does about him and his family. He advises that she stay away from him for her own safety, all the while denying his desire to keep her close at hand.
The seemingly blissful state of affairs is thrown into chaos when another vampire coven sweeps into Forks, and their leader, James, sets his sights on Bella. Thrilled as much by the idea of outwitting a clan of vampires as by Bella's scent, an interstate game of cat and mouse ensues. Bella, under the assumption that James has kidnapped her mother, is lured to her former ballet studio. At the studio, James tells Bella about Alice, his former prey. However, James' attention centers back on Bella, and attacks her. Edward rescues Bella before she is almost killed by James, along with the Cullen clan, save for Esme. In the chaos, Bella is nicked by James, which complicates matters further, due to the venom in the saliva when a vampire intends to bite. The venom works its way through the body, ceasing blood flow and shutting down the heart. This process of changing into a vampire is incredibly painful, as described by Carlisle. Edward takes up the task of sucking the venom out of Bella, thus saving her from the transformation, while also restraining his urge to drain or turn her. Bella then blacks out and wakes up in a hospital bed. She finds out that she has a broken leg, several broken ribs, among other injuries. There, she talks to Edward about the fight with James, including why Edward sucked the venom back, preventing her from becoming a vampire.
The book ends with the two going to Bella's prom. Beneath the surface, tension remains as Bella wishes for Edward to turn her into a vampire so the two can spend eternity together, while Edward refuses to do so. The reason for Edward's adamant decision not to turn Bella into a vampire is not revealed.