From Wikipedia

The story is told in three parts by the main character, Michael Berg. Each part takes place in a different time period in the past.

Part I begins in an unnamed West German city (Heidelberg) in 1958. After 15-year-old Michael becomes ill on his way home, 36-year-old tram conductress Hanna Schmitz brings him to her apartment and cleans him up before bringing him to his parents. He spends the next several months absent from school battling a pre-existing case of hepatitis.

On a subsequent visit to thank her for her help, he realizes he is attracted to her; embarrassed after she catches him watching her get dressed, he runs away, but he returns at a later date. After she asks his help retrieving coal from downstairs, he becomes dirty and she bathes him; then they engage in sexual intercourse. He begins returning to her apartment on a regular basis, and the two take part in an affair. They develop a ritual of bathing and having sex, before which she frequently has him read aloud to her, chiefly from works of German literature. Both remain somewhat distant from each other emotionally despite their physical closeness. Hanna also is at times physically and verbally abusive to Michael.

Months later, Hanna suddenly leaves without a trace. The distance between the two of them had grown while Michael spent more time with his school friends, and so he feels guilty and believes it was something he did that caused her departure. The memory of Hanna taints all his other relationships with women.

In Part II, eight years later, while attending law school, he is part of a group of students observing a war crimes trial. A group of middle-aged women who had served as guards at a satellite of Auschwitz near Kraków are being tried for allowing Jewish women under their ostensible protection to die in a fire at a church that had been bombed during the evacuation of the camp. The incident had been chronicled in a book written by one of the few survivors, who emigrated to America after the war; she is the star witness at the trial.

To Michael's surprise, Hanna is one of the defendants. This sends him on a roller coaster of complicated emotions. He feels guilty for having loved a criminal and is also mystified at Hanna's willingness to accept full responsibility for having supervised the other guards despite evidence proving otherwise. During the trial, Michael realizes that all her life Hanna has been protecting what is to her a more terrible secret than her Nazi past: she cannot read or write. A critical moment is when Hanna refuses to give a sample of her handwriting; at which point, if Michael had revealed that she could not write, then she was not guilty of writing the account of the church fire, in which case she would not have received a life sentence. This omission on Michael's part adds to the complexity of German guilt and is an important part of this review. This inability shaped all her actions, her original refusal of the promotion that put her in the position to directly kill these people, and also her panic the rest of her life over being discovered. During the trial, it comes out that she took the weak and sickly women and had them read to her before they were sent to the gas chambers. Michael decides she wanted to make their last days bearable; he later decides she sends them to the chambers so they won't reveal her secret. The reader is left to interpret her motives. She is convicted and sentenced to life in prison.

In Part III, Michael, trying to come to terms with his feelings for Hanna, begins taping readings of books and sending them to her without any correspondence. Years have passed, Michael is divorced and has a daughter from his brief marriage. Eventually Hanna learns to read by borrowing the books from the prison library and following along in the text. She writes to Michael, but he does not reply. When Hanna is about to be released, he agrees (after hesitation) to help find her a place to stay and gainful employment, visiting her in prison. On the day before her release in 1984, though, she commits suicide. Michael learns from the warden that she had been reading books by many prominent Holocaust survivors, such as Elie Wiesel, Primo Levi, Tadeusz Borowski, and histories of the camps. The warden is angry with him for not communicating with Hanna in any way other than the audio tapes.

In a dénouement, Michael visits the Jewish woman who wrote the book about the death march from Auschwitz and is now living in New York. She observes, for the first time in the story, how inappropriate Michael's and Hanna's relationship was, and how it damaged him, and draws parallels to Hanna's treatment of the poor and the weak at the camp. She insightfully asks whether he had a short, unloving marriage, and whether he had a child who was now away from him. She refuses to take the savings Hanna had asked Michael to convey to her, saying, "[Hanna] cannot buy my forgiveness so cheaply". She suggests he donate it to a Jewish charity of his choice. He chooses one that focuses on reducing adult illiteracy. The woman does, however, take the old tin tea box where Hanna had kept her papers and mementos, "to replace the similar tea box which she herself had until being sent to the camp" — a small ambiguous gesture towards her former guard. After this meeting, Michael visits Hanna's grave for the first and only time.