Edward left Bella in New Moon, which changed the course of her life, literally, forever. With her father, they travel to Italy, and are both changed by the Volturi, but at different times, as Bella is pregnant with Edward's children. After a lot of twists and turns, they are given an assignment after 30 years, which causes destruction within their family as the children meet up with those that left their mother behind, causing more tension in the town of Forks than what they were sent there for, in the first place.
Filled with dread, Bella knows she has to make a decision as the Queen of Volterra: go with her heart, or go against her loyalty, both choices that are the same in their own rights, but will decide the course of the future for not only Bella, the Volturi, the Cullens, Edward, or her daughters, but for humankind, itself.
P.S. Summary sucks, but hopefully you get the picture ;-) My ADHD wants me to say more than what I probably should on a lot of things, right now...LOL.