When we meet Melinda on the first day of her freshman year, we know that all her ex-friends and a bunch of other kids are really mad at her. She also hints that something awful has happened to her, and she wants to tell somebody about it but can't.
We find out that people are mad at Melinda because they hold her responsible for an end-of-summer party being busted. Melinda is physically and verbally assaulted at school on a semi-regular basis, prompting her to keep to herself more and more. We also learn that the bad thing happened to her at the party, and that's why she called the police. She is deeply conflicted about whether to talk about it or not.
Melinda could avoid facing her secret indefinitely if it weren't for the fact that Andy Evans goes to her school and has made contact with her. We strongly suspect in this stage that whatever bad thing was done to her at that party was done by Andy. He terrorizes her more and more and is starting to move on to her ex-best friend, Rachel. Now, she has to do something. But it's so complicated.
Melinda is finally motivated to speak and warns Rachel about Andy. Rachel doesn't seem to believe her at first, but Melinda's anonymous warning probably prepares Rachel to ward off Andy at the prom, which she does.
When Andy traps Melinda in her hiding place at school, he has a personal vendetta against her, unlike when he attacked her at the party. It seems like he will beat her or even kill her in addition to raping her if she doesn't escape his clutches. She does escape, though. Nothing like a shard of glass to the neck to put somebody in his place.
We don't learn what happens to Andy, but the secret is out, and he's hopefully in the hands of the police now. On the last day of school, Melinda sees a different side of Merryweather High, a side of Merryweather that's on her side. Classmates express sympathy, and we learn that Rachel has reached out to Melinda. Melinda also seems on the road to a better relationship with Mom and Dad.
After earning an A+ on her final art project, Melinda feels her throat loosening. She begins to confide her story to Mr. Freeman.