Dmitri Dmitritch Gurov has been hanging out for about two weeks in Yalta, on the coast of the Black Sea, when he hears of the arrival of a new vacationer, known only as "the lady with the dog." Gurov is 40, married, and has kids. He dislikes his wife and women in general, and has no qualms about having affairs. He is troubled only when his lover forms some sort of emotional attachment to him. The lady with the dog soon becomes his next conquest.
Her name is Anna Sergeyevna, and she, too, is married, though her husband is absent. She and Gurov quickly become friends and then lovers. Afterwards, Anna considers herself a fallen woman, and with shame assumes that Gurov will never respect her again. Gurov finds himself bored by her concerns. The affair continues, but ends abruptly when Anna is summoned back home by her husband.
Gurov returns to his home and family in Moscow and embraces his old life, hoping and assuming that he will forget all about Anna. But this is not the case. Consumed with thoughts of her, confused by his feelings, and suspecting love, he travels to Anna's house and seeks her out one night at an opera while her husband is outside smoking. Anna is horrified by his arrival and terrified that someone will see, but she admits that she hasn't been able to stop thinking about him since she left Yalta. She begs Gurov to leave before someone grows the wiser, and leaves him with the promise that she'll come to see him in Moscow.
Gurov returns home, and Anna follows through on her promise. She begins visiting Moscow semi-regularly, where she stays in a hotel and carries on her affair, if intermittently, with Gurov. Visiting her in the hotel one day, Gurov realizes that he is in love with her, and that this is the first time he's ever been in love. He has two lives, he concludes, one secret and valuable, the other public and worthless.
Anna is in tears as they both lament their plight: in love with each other but married to other people. They discuss and debate, trying to find a way to be together despite the circumstance. They know there is a long road ahead, and that the most difficult part is just beginning.