Anna's brother, Stepan, is married to Kitty's older sister, Darya. The book begins with a crisis in Stepan's marriage: he's been unfaithful to Darya. Ironically, Anna is the one who convinced Darya to forgive Stepan. Shortly thereafter, she meets Vronsky and carries out a flirtation with him that ends in adultery.
At first, Anna's affair is about her infatuation with Vronsky and his near-worship of her. But after Karenin discovers their affair, she chooses to run away with Vronsky, and a destructive chain of events is set in motion.