The novel begins as our narrator, Clay Jensen, mails a mysterious package to someone named Jenny. We soon learn that the package contains the audiotaped suicide note of Hannah Baker, a girl Clay had a crush on before she killed herself about two weeks ago. This is going to be a doozy, Shmoopers.
In the package, there are a total of seven cassette tapes and thirteen stories. On the first tape, Hannah tells her listeners that she holds each of them responsible in some way for her death, and that the tapes will explain why. After listening, each person must give the tapes to the next person on the list. She says that if anybody fails to pass them along, a copy of the recordings will be made public. The tapes also come with a map that listeners are meant to physically follow as they listen to her story.
The narrative moves back and forth between Hannah's and Clay's thoughts and actions. This is hard to portray in a short summary, so remember that Clay's thoughts are always mixed in with Hannah's stories. (Basically, go read it: it's a really interesting, challenging, and pretty brilliant structure.)
The thirteen stories (i.e., the "reasons why") are as follows: (1) Justin Foley, Hannah's first kiss, started rumors that they did more than just kiss. (2) Alex Standall pitted Hannah and Jessica Davis against each other by voting Hannah "Best Ass of the Freshman Class." (3) Jessica Davis was upset by this and ended up hitting Hannah, leaving a scar on her forehead. (4) Tyler Down allegedly took pictures of Hannah through her window (she calls him a Peeping Tom). (5) Courtney Crimson, a super fake girl at school, started rumors that Hannah had sex toys in her bedroom.
As he's listening to Courtney's tape, Clay gets on the city bus and runs into his middle school crush, Skye Miller. He starts to wonder why Skye always isolates herself from everybody. They exchange a few words and then Clay gets off the bus, outside of Tyler Down's house. There, he runs into Marcus Cooley, who invites Clay to throw a rock at Tyler's window (which is already broken). He learns that Marcus is also on the tapes, so he must be guilty of something, too.
Back to the tapes: (6) After getting matched up with Hannah through a Valentine's Day survey, Marcus tried to make a move on her in a booth at Rosie's Diner. She had to push him out of the booth and onto the floor to get him to stop.
Still listening, Clay heads toward Rosie's (next stop on the map); on his way he sees the Crestmont Theater, where he and Hannah worked the previous summer. He got the job to be close to her, but he always let the rumors about her stand in the way of telling her how he felt.
More recordings: (7) When Hannah ignored Zach Dempsey's attempts to comfort her after the incident with Marcus, Zach stole the notes of encouragement left for Hannah by classmates in her Peer Communications class. (8) Hannah met Ryan Shaver in a poetry class and really trusted him; that is, until he stole a very personal poem of Hannah's and published it, leading to more ridicule for Hannah.
Clay leaves Rosie's and finds his friend Tony waiting for him in the parking lot. Tony tells him that he is the one who has the second set of tapes, the ones that will be released to the public if Clay's tapes aren't passed on according to Hannah's wishes. Clay is full of questions, but Tony refuses to answer anything until Clay listens to the next recording.
So, the tapes: (9) The ninth tape features Clay and assures him that he had nothing to do with Hannah's suicide. He was actually really good to Hannah; he even confessed his feelings to her and they kissed! (10) Justin Foley, from the first tape, is back on the recordings. This time, Justin allowed Bryce Walker to rape Jessica while she was unconscious.
At this point, Tony explains that on the day of Hannah's death, he found the tapes waiting for him when he got home from school. When he realized what Hannah was up to, he called her parents to try to warn them, but he was too late. Since then, he's been following the people on the list making sure they follow Hannah's instructions.
One last dive into the tapes: (11) After a party, Jenny Kurtz offered Hannah a ride home, but knocked down a stop sign with her car. Jenny refused to report the incident, and shortly after, there was a fatal accident at the stop sign. (12) One night, Bryce Walker started to touch Hannah in a hot tub, and he proceeded to rape her. (13) Mr. Porter, Hannah's guidance counselor, didn't take her seriously when she said she was suicidal, and he let her leave his office without getting her any help.
The novel ends with Clay going to school the next day after mailing the tapes to Jenny. When he gets there, he sees Skye Miller, the girl from the bus. He walks toward her, saying her name.