The Oblonsky family of Moscow is torn apart by adultery. Dolly Oblonskaya has caught her husband, Stiva, having an affair with their children’s former governess, and threatens to leave him. Stiva is somewhat remorseful but mostly dazed and uncomprehending. Stiva’s sister, Anna Karenina, wife of the St. Petersburg government official Karenin, arrives at the Oblonskys’ to mediate. Eventually, Anna is able to bring Stiva and Dolly to a reconciliation.
\Meanwhile, Dolly’s younger sister, Kitty, is courted by two suitors: Konstantin Levin, an awkward landowner, and Alexei Vronsky, a dashing military man. Kitty turns down Levin in favor of Vronsky, but not long after, Vronsky meets Anna Karenina and falls in love with her instead of Kitty. The devastated Kitty falls ill. Levin, depressed after having been rejected by Kitty, withdraws to his estate in the country. Anna returns to St. Petersburg, reflecting on her infatuation with Vronsky, but when she arrives home she dismisses it as a fleeting crush.
Vronsky, however, follows Anna to St. Petersburg, and their mutual attraction intensifies as Anna begins to mix with the freethinking social set of Vronsky’s cousin Betsy Tverskaya. At a party, Anna implores Vronsky to ask Kitty’s forgiveness; in response, he tells Anna that he loves her. Karenin goes home from the party alone, sensing that something is amiss. He speaks to Anna later that night about his suspicions regarding her and Vronsky, but she curtly dismisses his concerns.
Some time later, Vronsky participates in a military officers’ horse race. Though an accomplished horseman, he makes an error during the race, inadvertently breaking his horse’s back. Karenin notices his wife’s intense interest in Vronsky during the race. He confronts Anna afterward, and she candidly admits to Karenin that she is having an affair and that she loves Vronsky. Karenin is stunned.