Eclipse opens with a preface in which Bella and "her protector" in great peril. They're trapped by an unknown enemy. We learn that a war is raging somewhere, but that Bella doesn't believe she'll live to learn the outcome.
Flashback to Bella's home in Forks, Washington. She reads a letter from Jacob, a werewolf, in which he tells her that they can't be friends anymore. She misses him. Bella's father Charlie urges her to hang out with some friends other than her (vampire) boyfriend Edward – Jacob, for example.
Charlie informs Bella and Edward of a killing spree in Seattle. Edward reveals to Bella that his family thinks bloodthirsty newborn vampires have committed the killings.
Jacob becomes a sore point for Edward and Bella. Bella demands to see Jacob, but Edward sabotages her attempts to run off to see him.
Edward convinces Bella to visit her mom in Florida. After their return, Jacob shows up at Forks High and reveals to Bella that evil vampire Victoria, who is set on killing Bella, has returned. The hunt for the bad vampire has caused a run-in between the Cullens, Edward's good vampire family, and the Quileute werewolves, including Jacob. Bella makes Edward promise to stop being so overprotective of her.
Bella escapes the supervision of Edward's sister Alice and sneaks off to see Jacob. When she returns, Edward is livid, but she's had enough of the hostilities between vampires and werewolves. "I am a neutral country. I am Switzerland," Bella says.
Rosalie, Edward's other vampire sister, tells Bella her life story and her dream of having a baby. Becoming a vampire against her will destroyed that dream forever. She urges Bella to not give up her mortality.
Jacob appears on a motorcycle at Forks High to "kidnap" Bella from Alice's supervision. Bella seizes the moment and takes off with him. Jacob tells Bella that he'd be a better choice for her than Edward, but Bella tells him she loves Edward. He clues Bella in to "imprinting," which is like love at first sight for werewolves. Some of his friends have already imprinted, but he hasn't yet. When Bella reveals that she'll be transformed into a vampire after graduation, Jacob loses his temper and says he'd rather see her dead.
When she returns from seeing Jacob, Bella is surprised to hear that Edward has decided to allow her to visit Jacob to show his trust in her.
When Bella notices that some of her clothes are missing, Edward confirms that an unknown vampire has been snooping around her house. Jacob arrives to pick up the unknown scent for the werewolves to track. Edward tells Bella that the killing spree in Seattle has worsened, leading the Cullens to believe that someone is building an army of vicious newborn vampires.
With graduation just one week away, Bella realizes she will soon be transformed into a vampire and gets cold feet. She still wants Edward to change her, but she cringes at the idea of marrying him, because she's concerned about what her parents and the people in Forks will think of her.
During a gathering of the Cullen family regarding the newborn vampires all over Seattle, Jasper, Edward's brother, reveals that he was transformed into a vampire to serve in an army, created by a vampire named Maria, to expand her feeding territory. He explains that newborn vampires have enormous strength, but aren't yet skilled fighters. The Cullens decide to stop the newborn vampires in Seattle before the Volturi, a powerful group of Italian vampires, do so, because their visit might lead them to check if Bella is still human. (Check out our summary of New Moon if you need a refresher on Bella's interactions with the Volturi.)
Jacob declares his love to Bella. He forces her into a kiss. She punches him and breaks her hand in the process. At Bella's house, Edward confronts Jacob. Their fight for Bella is officially on. "May the best man win."
Bella realizes that her earlier unknown vampire visitor and the vampire army are connected. She believes that someone is creating a vampire army just to kill her.
Jacob and his werewolf buddies Embry and Quil show up to Bella's graduation party at the Cullen house. Bella forgives Jacob for kissing her. When Alice, who can see the future, has a vision of the vampire army marching toward Forks, the werewolves and Cullen vampires form an alliance against their common enemy. They meet in the woods where Jasper tells both his family and the werewolves how to kill newborn vampires. Bella makes Edward promise to stay with her, because she can't stand losing him. He agrees.
Edward and Jacob come up with a plan to hide Bella from the vampire army in the woods, far away from the scene of battle. Jacob decides to have his cousin Seth, a young werewolf who's new to the pack, guard her during the fight.
Bella confides to Edward that the one human experience she wants to have before her transformation is to make love to him. Edward refuses for fear that he might hurt or even kill her. After a heated argument, they come to a compromise: Edward will try, but only after Bella marries him. He gives her an engagement ring and proposes.
The night before the battle, Edward, Bella, and Jacob decide to sleep in a tent in the woods. A snowstorm rages outside. Bella is freezing to death, so Edward (whose skin is always ice cold) grudgingly agrees to let Jacob warm Bella. Edward and Jacob have a heart-to-heart in which Edward says that he'd let Bella go without a fight if she chose Jacob.
Moments before the battle, Jacob learns that Edward and Bella are getting married. He's crushed and tells Bella that he's going to kill himself in battle. Bella pleads with him. They kiss. This time, Bella loses her self-control and realizes that she's in love with Jacob.
Bella feels horribly guilty, but Edward forgives her. The fight goes smoothly, until – out of the blue – Victoria and her companion vampire Reilly appear. Edward takes on Victoria. Seth attacks Reilly. It's a close fight but, in the end, Edward and Seth prevail by working as a team. The Volturi arrive at the scene of battle, awed that all their work has been taken care of by the Cullens (as they don't know about the existence of the werewolves).
Bella visits Jacob, who's been wounded in battle. He promises Bella that he'll let her go, but that he'll always be waiting in the wings – maybe even after she turns into a vampire.
After leaving Jacob, Bella experiences an emotional breakdown. Yet she believes she's made the right choice. Though she's also been upset about getting married, she makes peace with the idea, because she feels that she owes it to her parents to share this important human tradition with them before becoming a vampire. She even agrees to let Alice plan a big wedding.
When Jacob receives Bella's wedding invitation, he's devastated, Even more, he's enraged. He turns into wolf form and disappears into the woods, thinking to himself, "I would never go back…I pushed my legs faster, letting Jacob Black disappear behind me" (Epilogue.86).