Speak is a first-person nonlinear narrative written from the perspective of the protagonist, Melinda Sordino. When the story begins, Melinda is starting her first year at Merryweather High School in Syracuse, New York. She remarks that Rachel, Nicole, and Ivy—her former friends from middle school—are no longer speaking to her and have joined new cliques over the summer holidays. Having spent the last weeks of summer break alone and unhappy, Melinda is barely able to speak to anyone on her first day. She is already counting the days until she will be able to graduate and finds that there are 699 of them. Her classes are boring and pointless, except for art, taught by the kindly and inspiring Mr. Freeman. Mr. Freeman explains that each student will create art projects around a single theme throughout the year. Melinda’s assigned theme is “tree.”

Melinda’s home life is no better than her time at school. Her mother and father are preoccupied by their work and communicate with her mainly by leaving notes. She spends most of her time in her bedroom, which she dislikes because it was decorated when she was in fifth grade and looks too childish for a high school student. Melinda’s bedroom also reminds her of a time when she had friends. Rachel, once her closest friend, now hates her and only wants to spend time with her new clique of foreign exchange students. Melinda’s only friend is Heather, a new girl who befriended her on the first day; however, Melinda does not feel particularly close to Heather and finds her enthusiasm and positive attitude annoying.

When Melinda and Heather attend the Homecoming pep rally together, another student recognizes Melinda as “the one who called the cops at Kyle Rodgers’s party” over the summer. This turns everyone against Melinda, and the other students start to bully her. Later, when Heather wants to get more involved in school activities and suggests performing in a musical, Melinda tells her that there is no place in such projects for outsiders like them. The two quarrel over Melinda’s negativity. Melinda becomes steadily more apathetic and achieves low grades, upsetting her parents. As she is too old to go trick-or-treating and no one has asked her to a party, she spends Halloween alone, reading Dracula.

Heather wants to join a clique called “the Marthas” and Melinda helps her to decorate the faculty lounge for Thanksgiving, a project the Marthas have undertaken. However, when they have finished, the Marthas sneer at Melinda, send Heather away, and pretend they have done all the decorating themselves. Soon after this, Melinda sees a boy to whom she refers only as IT in the corridor, and freezes with terror. IT winks at her, and she feels as if her “lips are stitched together.”

Melinda finds refuge from school in an abandoned janitor’s closet, which she decorates to her own taste with a poster of Maya Angelou. As she becomes more isolated, she spends more time in the closet cutting classes. She retreats further inward, finding it even more difficult to speak than she did at the beginning of the semester, particularly in front of teachers. Her mother tries to bring the family together by cooking a turkey at Thanksgiving, but she fails even to thaw it properly and they end up eating store-bought donuts and pizza. Melinda takes the turkey bones to school and makes a tree sculpture with them in Mr. Freeman’s art class.

Melinda remains silent in class but finds inspiration in the way her classmate, David Petrakis, stands up to their bigoted social studies teacher, Mr. Neck. David starts bringing a tape recorder and even a video camera to class to record Mr. Neck’s inappropriate comments and conduct. After a lonely Christmas, Melinda returns to school in the new year to find nothing changed. Art class is still the only redeeming feature of school, and Heather is still desperate to join the Marthas, who only take advantage of her.

Melinda faints and hits her head on the table while dissecting a frog in biology class. It is clear that the dissection has triggered a traumatic memory, and the trauma is exacerbated by a subsequent encounter with IT, who comes up to her and whispers “Freshmeat” in her ear. Afterward, she scratches her wrist with a paperclip until she draws blood, but when her mother sees this, she coldly tells Melinda that “suicide is for cowards.” Later, in the art room, Melinda cuts her thumb with a chisel. Mr. Freeman, who is deeply depressed by the state of his career, uses the same chisel to slash a canvas on which he has been working.

The next time she sees IT, whose name has finally been revealed as Andy Evans, at a bakery, he offers her a bite of his donut and grins wolfishly at her. Melinda runs away and decides to miss school that day. Heather informs Melinda that she does not want to be her friend any longer and thinks she should seek professional help. As Melinda’s behavior deteriorates and she misses school again, her parents have a meeting with the school’s ineffectual principal. Mr. Sorvino recalls that the year before, Melinda had no problems and blames the principal and school for her current state. She is forced to attend in-school suspension alongside Andy Evans, whom she dreams of killing.

Melinda’s behavior improves, and she is particularly interested in her study of Picasso in Mr. Freeman’s art class. One day, David Petrakis invites her to join him and his friends for pizza at his house. Although she likes David, she freezes in fear and declines. Later that night, she has a flashback in which she recalls what happened at Kyle Rodgers’s party over the summer. Andy Evens danced with her, told her that she was beautiful, and kissed her. Suddenly, he was on top of her, and she was trying vainly to protest, as he covered her mouth. After Andy raped her, Melinda made her way back to the party in a daze and called 911 but found herself struck speechless, unable to tell the operator what happened. Her silent call led the cops to arrive and break up the party. She recalls vague flashes from the rest of that night: red and blue lights flashing; someone slapping her; teenagers panicking as they tried to flee; her friends yelling at her. When she emerges from her memories and back into the present day, Melinda realizes that she has bitten through her lip.

Some time after this flashback, Melinda discovers that Rachel has gone to see a movie with Andy. Although she is still angry with Rachel, she worries that Andy may harm her, and so she eventually writes an anonymous note to warn her former friend of his true nature. She is performing better at school now and writes a good social studies report for Mr. Neck. However, the teacher is aware of her reluctance to speak and sadistically demands that she deliver the report orally. Instead, Melinda writes a note on the board, which says that just as the suffragettes fought for the right to speak, she is fighting for the right to be silent. Mr. Neck sends her to in-school suspension.

Melinda is working in the art room, one of the few places where she feels safe, when Andy enters and turns the lights off, terrifying her. She freezes with fear, but is saved when Rachel comes in, looking for Andy. The next day, she tells her mother she is sick and misses school. Melinda begins to reconnect with Ivy, one of her former friends. Ivy remarks that she thinks Rachel is stupid for dating Andy Evans, who she deems “trouble with a capital T.” She and Melinda add to the gossipy graffiti in the girls’ bathroom, scribbling a warning that Andy is a guy to “stay away from.” 

Heather visits Melinda at home, but it quickly becomes clear that she is just reaching out not because she misses her, but because the Marthas have tasked her with setting up the prom decorations and she wants help. Feeling bold, Melinda finally stands up to Heather and tells her to leave. Feeling empowered after this encounter, Melinda finally decides to tell Rachel that Andy raped her. Rachel is sympathetic upon hearing that Melinda was raped but becomes furious when Melinda accuses Andy and refuses to believe that he would do such a thing. Rachel’s reaction leaves Melinda disheartened, but she feels better when Ivy shows her the graffiti they drew in the bathroom. Other girls have since added their own messages, calling Andy “a creep” who should be “locked up.” Melinda is later overjoyed to hear that Rachel broke up with Andy at prom.

As the school year comes to a close, Melinda, feeling better than she has in a long time, decides she no longer needs her hiding spot in the janitor’s closet. When she enters the closet to clean it out, however, Andy follows her inside. Furious that the girls in school are treating him like a pariah, Andy accuses Melinda of spreading rumors about him. He attacks her, intending to rape her again. Melinda screams “loud enough to make the whole school crumble” and attempts to fight him off; they struggle until she finds a broken shard of  glass and holds it to his neck, drawing blood. Andy freezes, paralyzed with fear and unable to speak. At this point, Melinda hears someone knocking on the door and finds that a group of girls from the lacrosse team heard her screams and have come to help her. The lacrosse girls tell the rest of the school about Andy’s attack, and Melinda finds that she is suddenly a popular figure, attracting widespread sympathy. Rachel reaches out as well, asking Melinda to call her. 

On the last day of school, Melinda stays late in her art class, determined to get her final tree project just right. She draws a tree that is imperfect. Its lower branches are sick, but its crown is made up of strong, healthy new growth that reaches up for the sun; this, Melinda says, is the best part. As she finishes her tree, Melinda finally confronts her trauma, promising herself that “I’m not going to let it kill me. I can grow.” Mr. Freeman comes over and awards Melinda an A+ for her tree, remarking that it seems like she’s been through a lot. Finding her voice, Melinda answers, “Let me tell you about it.”