Thirteen Reasons Why is a Young Adult novel that tells the story of Clay Jensen, a high school student who returns one day to find a box of cassette tapes recorded by Hannah Baker, his classmate who killed herself earlier that year. As he listens to the tapes, Clay realizes that they are Hannah's suicide note, recounting thirteen experiences that led her to become suicidal. As Clay listens he is overcome with the realization of how our actions affect those around us, and how important it is to reach out to those who may be struggling.
Clay Jensen, a shy high school student, returns home from school one day to find that he has received a mysterious package in the mail. The package is revealed to contain thirteen cassette tapes recorded by Hannah Baker, a classmate who recently committed suicide. Before her death, Hannah explains that thirteen people, including Clay, are to blame for her depression. The tapes, which detail how each person drove her closer to suicide, are to be passed between the thirteen people, in the order they are mentioned. Hannah claims that she has given a second set of the tapes to an unnamed classmate, who will leak them to the whole school if the tapes do not reach the thirteenth person.
The first person to receive the tapes was Justin, whom Hannah met shortly after she first moved to town. They kissed once, after Hannah developed a crush on him. However, Justin told his friends that they did more than just kiss, which earned Hannah the reputation of being a slut at school.
The second person to receive the tapes was Alex, with whom Hannah was once friends. Alex violated their friendship when he published a "hot or not" list comparing the girls in their class. He awarded Hannah the title of Best Ass, which only reinforced her reputation as a slut. He also awarded his ex-girlfriend Jessica the title of Worst Ass, in revenge for her breaking up with him.
The third person to receive the tapes was Jessica, with whom Hannah was once friends. After being compared to Hannah on the "hot or not" list published by Alex, Jessica slapped Hannah and effectively ended their friendship. The slap left a scar on Hannah's forehead, which acts as a physical manifestation of the pain she feels inside.
The fourth person to receive the tapes was Tyler, a classmate who worked as a photographer for the yearbook. After suspecting that Tyler was taking pictures of her through her bedroom window, Hannah enlisted their classmate Courtney to help her catch Tyler in the act. The two girls hung out in Hannah's bedroom one night and, at Courtney's insistence, gave each other sensual massages in order to excite Tyler. They then snapped open the curtains to see Tyler running away from the window. Hannah blames Tyler for making her feel unsafe in her own bedroom.
The fifth person to receive the tapes was Courtney, who used Hannah in order to make herself more popular. Known as the sweetest girl in school, Courtney revealed her true nature to Hannah when she spread a rumour regarding the sensual massages they gave each other in Hannah's bedroom, which only reinforced her reputation as a slut.
The sixth person to receive the tapes was Marcus, who believed the rumours about Hannah and tried to take advantage of her as a result. After being paired up by a Valentine's Day fundraiser at school, Marcus asked Hannah on a date to a local diner. Ignoring her protests, Marcus repeatedly tried to touch Hannah inappropriately. When she finally pushed him away in anger, Marcus called her a tease loudly enough for the entire diner to hear. It was then that Hannah, embarrassed for thinking Marcus actually liked her, started considering suicide.
The seventh person to receive the tapes was Zach, who tried to comfort Hannah after Marcus stormed out of the diner (though Hannah later realizes Zach was only acting on a dare). Later, in their Peer Communications class, each student is given a pouch in which their classmates can leave anonymous compliments. To spite Hannah for rejecting him at the diner, Zach steals all of the compliments Hannah has received before she can read them. When Hannah catches him, Zach flees without apologizing. This leads Hannah to leave their Peer Communications teacher an anonymous note confessing her suicidal thoughts. The teacher asks the class to discuss the note, and most of the students respond with annoyance.
The eighth person to receive the tapes was Ryan, the editor of the "Lost N Found" column in the school newspaper. After gaining her trust, Ryan stole a poem written by Hannah in which she detailed many of her personal problems. He published the poem anonymously in the school newspaper, but some of their classmates realized that Hannah wrote it and proceeded to mock her. Hannah blames Ryan for making her feel like even her thoughts are not safe from scrutiny.
The ninth person to receive the tapes was Clay. Hannah apologizes for including Clay on her list, as he did not make her suicidal like the rest. In fact, she admits that Clay was the nicest person she ever met and that she wishes they had had more time to get to know each other. She recounts the night they hung out at a house party. After talking for hours, they went into a bedroom and started making out. Afraid that Clay would betray her just like Justin did, Hannah started shouting at Clay to leave her alone. After trying to comfort her, Clay eventually gave up and left her alone in the bedroom, where she began to cry. Hannah apologizes for her behaviour and expresses hope that the tapes will explain her motives to Clay.
While listening to the tapes, Clay walks around town and visits the various places mentioned by Hannah. He runs into their classmate Tony, who acts strangely upon seeing the tapes. Clay realizes that Tony must be featured on the tapes too; he demands to know what Tony did to make Hannah suicidal, but Tony only tells Clay to keep listening to the tapes.
In shock, Clay again demands to know what Tony did to make Hannah suicidal. Tony admits that he is not mentioned on any of the tapes, and that he is the unnamed classmate to whom Hannah gave the second set of tapes. Out of disgust for everything mentioned on the tapes, Tony admits that he will indeed leak them to the whole school if the tapes do not reach the thirteenth person. Clay leaves Tony to his thoughts, continuing his walk around town while he starts the next tape.
The tenth person to receive the tapes should be Justin, although Hannah acknowledges that he will have already received the tapes once and that Clay should pass them on to Jenny instead. Hannah describes hiding in the closet after Clay left her alone in the bedroom at the house party. Before she could leave, Justin and a friend of his dragged a drunk Jessica into the bedroom. The friend proceeded to rape Jessica while Justin guarded the door. Hannah expresses anger at both Justin and herself for not speaking up.
The eleventh person to receive the tapes will be Jenny, a cheerleader who offered Hannah a ride home from the house party. After Jenny crashed and knocked down a stop sign, Hannah tried to call the police. Not wanting to get in trouble, Jenny kicked Hannah out of her car and fled the scene. Two cars crash later that night, as a result of the stop sign being knocked down. Another classmate is killed in the crash, but neither Jenny nor Hannah step forward to take the blame.
The twelfth person to receive the tapes should be Bryce, although Clay doubts if Bryce would care enough about the tapes being leaked to pass them on himself. Hannah describes house-sitting one night while Courtney threw a house party down the street. Remembering the disastrous events of the last house party she attended, Hannah waited until most people had left before she wandered over. There, she joined Courtney and Bryce in the hot tub. Hannah describes letting Bryce pressure her into having sex with him, while Courtney turned a blind eye. Clay reads between the lines and realizes that Bryce was the one who raped Jessica, which is the real reason why Hannah included him on her list.
The thirteenth person to receive the tapes will be Mr. Porter, the counselor at school. Hannah secretly recorded a conversation they had in his office, in which she expressed a desire to kill herself. Overwhelmed by the situation, Mr. Porter simply told Hannah that if she was unwilling to press charges against Bryce, then she should just try to move on. Hannah fled his office then, clearly set on committing suicide, and Mr. Porter did not go after her. Hannah says "I'm sorry" before the tape cuts off. On the other side of the tape, Hannah records herself saying only two words: "Thank you."
Clay returns to school the next day and finds himself unable to stop thinking about Hannah. He runs into his classmate Skye, whom he has suspected of being suicidal. The novel ends with Clay reaching out to Skye.