Clay Jensen doesn't know why Hannah Baker committed suicide, but he's about to find out. Before she died, Hannah recorded a set of cassette tapes, one side for each of the 13 reasons she chose to take her life. Each reason relates to a person. Each of those 13 people must listen to her tapes, then pass the set on to the next person. And Clay is one of them.
When the tapes arrive in the mail, Clay is confused. Aside from making out with Hannah at a recent party, he didn't know her well. Despite her loose reputation, he really liked her, and he would never have done anything to hurt her. Surely there was a mistake; he can't be responsible for Hannah's suicide. But Hannah has threatened that the tapes will be made public if he doesn't follow her instructions. So he pops the first tape into an old stereo system in his parents' garage.
The first name on the tapes is Justin Foley, the first boy Hannah kissed. Clay knows him as an average guy who is popular with the girls. He also knows that Justin and Hannah did more than just kiss — or did they? According to the tapes, the rumors were false, spread by a boastful Justin. To Justin, the rumors meant nothing. But for Hannah, a newcomer, those rumors were the beginning of a reputation that would be impossible to shake.
Clay is hooked on Hannah's story, but he doesn't want to keep listening in the garage where his family could walk in at any time. Under the pretense of helping his friend Tony start his less-than-reliable Mustang, he steals Tony's Walkman. Hannah has also left Clay a map, starred with significant locations that she will mention throughout the tapes. Clay walks to the first location while listening to the next tape.
The first star on the map is Hannah's old house, now the home of an elderly man who was involved in a serious car accident earlier that year in which a high school student from Clay's school was killed. The second name is Alex Standall, the boy who named Hannah "Best A--" in the freshman class. While Alex may have meant it as a twisted compliment, it only served to spiral Hannah's reputation even lower. Boys now felt that they had the right to touch her whenever they felt like it. Clay remembers the girl who was voted Best Lips. He was mesmerized by them and even made out with her — all because of the list.
The third name is Jessica Davis, another newcomer who was friendly with Hannah until more false rumors (this time about Hannah and Alex) caused Alex and Jessica to break up. At the location of the third star (a coffee shop), Jessica scratched Hannah's face, leaving behind a fingernail and a crescent-shaped scar.
The fourth name is Tyler Down, a peeping Tom who waited, armed with a camera, outside Hannah's window. Hannah invited Courtney Crimsen (fifth name) over to help her catch the crook. Courtney cared more about using Hannah than she did about being friends. Courtney got Hannah to drive her to a party, then ditched her. And at that party, Courtney started new rumors about Hannah.
On the bus ride to Tyler's house (fourth star), Clay runs into Skye Miller — his middle school crush. Over the past few years, she has isolated herself from the world, and Clay wonders why, but he gets off the bus without really talking to her. When he reaches the house, Tyler's window is broken and duct-taped, and Marcus Cooley, Alex's friend, is there with a rock. Clay refuses to throw it.
Listening to the sixth story, Clay learns that Hannah was Marcus' "soul mate" on the Oh My Dollar Valentine survey. Marcus asked Hannah for a date at Rosie's (next star), then almost stood her up. When he finally arrived, he groped her, even after she asked him to stop.
The seventh tape is the story of how Zach Dempsey stole notes of encouragement from Hannah during their peer communications class, and how the class refused to help when Hannah made an anonymous comment about considering suicide.
The eighth story is about Ryan Shaver, who stole one of Hannah's poems and exposed it to public ridicule. Clay listens and continues to travel to each starred location — now with Tony's help. Tony knows that Clay stole his Walkman, because Tony is the one Hannah asked to make the tapes public if her instructions weren't followed. Clay thinks back to the times he was too afraid to ask Hannah out, and wonders if things would have been different if he'd only had the courage to speak to her.
The ninth story is Clay's. Right away, Hannah admits that Clay doesn't belong on the list, that he didn't do anything wrong. She just wants him to hear her reasons and to know why she committed suicide. Hannah talks about how much she liked him and about the party where they lay on a bed and kissed and how she pushed Clay away because she was so afraid of getting hurt.
The 10th story is about Justin again. After Clay left the room but before Hannah could make her exit, Justin Foley and Jessica Davis lurched in. Hannah hid in the closet. Jessica was so drunk that she passed out before anything could happen. Justin left, but then reluctantly allowed his friend, Bryce Walker, to go into the room and rape Jessica. Hannah did nothing to stop it.
When Hannah wandered back into the party, she was visibly shaken. Cheerleader Jenny Kurtz (11th story) offered to drive her home. It was raining, and Jenny knocked over a stop sign. Hannah tried to convince her not to drive the rest of the way home and to report the downed sign. But Jenny refused. Hannah walked to a nearby gas station to call the police, but by the time she got there, it was too late — an accident had already occurred on the corner that was missing a stop sign. It was the accident involving the elderly man who lived in Hannah's old house.
The 12th story belongs to Bryce Walker. Hannah, spiraling fast, was determined to make some bad choices. At an after party, Bryce asked Hannah to join him and Courtney in a hot tub — in their underwear. Courtney left, and Bryce started groping Hannah. She didn't resist. He had sex with her. On the tapes, Hannah says that she was finally giving in to her reputation and that she was using Bryce to let go of herself.
The last tape, the 13th story, is about the school guidance counselor, Mr. Porter. Already determined to commit suicide, Hannah hopes he can talk her out of it. But he fails to get Hannah to open up to him, and she leaves. He doesn't follow.
It's now early morning, and Clay is finally done listening to the tapes. He mails them to Jenny Kurtz. The next day at school, Clay sees the world in a new light. He realizes that Skye is probably headed down the same road as Hannah. Clay is determined not to let that happen. When he sees Skye in the hallway, he walks toward her and says her name.