Peach Girl tells the story of Momo Adachi, a member of her high school swim team. She tans easily and her hair is bleached out; unfortunately, she is stereotyped by her ignorant classmates and is forced to endure rumors about being an "easy girl" who has had many sexual relationships. Her only friend is Sae, who is actually responsible for spreading the nasty gossip due to her jealousy of Momo. Momo is in love with Toji, a taciturn baseball player, but the scheming Sae also has her sights set on the boy. Momo's life is further complicated with the introduction of Kairi Okayasu, a wise-cracking playboy who is determined to make her his. He reveals to Momo that she had saved him from drowning the summer before high school started.
Things become even more complicated when Sae, in an attempt to make Momo miserable, spreads rumors about Momo and Kairi making out. The rumor about the kiss between the two is true, but it was Kairi who initiated it while Momo remained stunned by the interaction. When Toji is hospitalized, Sae convinces Toji to break up with Momo. Thanks to Kairi, he finds out about all the lies and rumors Sae has made up. Toji saves Momo from being bullied (thanks to Sae's lies again) and tells the truth about him and Momo, resulting in the students apologizing to Momo and hating Sae. Toji and Momo get back together, but Sae later tries to complicate their relationship again by threatening to discredit Momo with her newly acquired access to the mass media thanks to a male model named Jigoro. Though Toji is initially defiant, he reluctantly breaks up with Momo to protect her. Momo is heartbroken at first, particularly due to not knowing that it is part of Sae's schemes. However, she eventually begins dating Kairi, whom she had previously considered annoying, and soon begins to fall in love with him.
After the two start dating, his insecurities about relationships are revealed to stem from his unrequited love for Misao, the school nurse and Kairi's former tutor. Then a little later, Momo encountered Kairi's older brother Ryo, who is more dangerous than Sae. He tries to seduce her, but when he failed, he set his twisted eyes on Sae who fell in love with him and broke up with Toji for. Then one day, Ryo tricked Sae into starring in a pornographic movie which he said that if she didn’t want to then it was over between them. Sae called Momo for help and with the help of their classmates, they saved Sae. Sae fell more deeply in love with Ryo because of their obvious similarities. The next day, she apologized to Momo and Kairi and begins to repent for the horrible things she had done to them.
However, after Kairi confesses and is shot down by Misao, he actually feels relieved and confident to pursue his relationship with Momo, but decides to go for a walk to sort things out before going forward. Momo told him to meet her near the end of summer vacation to give her an answer. When Sae revealed to Momo why Toji broke up with her, Momo is left to choose between Toji and Kairi. More problems arise and she is unsure which boy she wants. When Kairi arrived, he found Momo and Toji together and thought that she had moved on. Heartbroken, Kairi along with Sae, went to the beach to yell out his heartbreak in the ocean, but Sae used one more trick to get them back together. By faking a text from him, Sae made it look like that Kairi was going to commit suicide in the ocean. Momo couldn’t stop thinking about him even when she was out with Toji so after some thinking and with Toji support, Momo went to the beach and in the end they get back together.