When Marissa is five, her mommy tells her that people are moving next door. Marissa has been waiting for this moment for a long time, and hopes that the people are like her family; a mommy, daddy and a girl her age. She has Summer, but she wants someone next door to share Barbies with and go on picnics with tea parties. Instead, it's Daddy's old friend, Kirsten, and her husband Sandy, and their five year old son Seth. Seth is funny in that way that's really weird and he doesn't talk to anyone. He complains about Newport and when Marissa asks him to play Barbies with him, he says no, but says he has a horse. Marissa has always wanted a horse and is excited. Then she finds out he means Captain Oats, who is a plastic horse. Marissa is disappointed and goes home, saying Seth Cohen is a liar and she won't talk to him.
When Seth is seven, Marissa comes down with the chicken pox one day. He asks if there is anything he can do for her because Kirsten and Sandy suggested it. Julie Cooper, who is Marissa's mom and really scary, suggests that he make sure he doesn't get it either. Seth is convinced the chicken pox isn't good, but goes to pick flowers that Jimmy or Julie can give to her. He picks the best ones and drops them off at Marissa's front steps. He doesn't want to wait for Jimmy or Julie to pick them up. He doesn't want them to think he has a crush on her. Girls are weird to Seth, but Marissa is the girl next door, she's different.
When Marissa is nine, she meets Holly. Holly is everything that Marissa wants to be. She is dangerous in that sense that makes Marissa realize what her mom has been saying about not growing up too fast. Holly makes enemies fast and dismisses those she deems so very quickly. Marissa is shy, her best friend being her little sister, but Holly has now chosen her to be her best friend. Seth notices Marissa's friend and wants to go over and introduce himself, because he thinks it would be good to be friendly and less quiet this year (or so he's heard his mom say), but he thinks it would be uncomfortable. Especially because Marissa barely talks to him nowadays.
When Seth is eleven, he's sent to the back of the bus during a field trip to the Museum of Tolerance because the teacher, Mrs. Schwartz, is sick of him provoking the other kids into a fight. Seth protests that they were the ones fighting, but Mrs. Schwartz asks who started it and they all say Seth did. Seth goes to the back of the bus and notices Marissa is making out with Luke, who's mean and yet Marissa likes him. He tries not to look, it's almost as bad as what his mom and dad do, but he starts wondering what would happen if it was him instead of Luke.
When Marissa is thirteen, she's barely around anymore. Seth knows if he was friends with her, he would ask what was going wrong, but he can't. But his mom says Marissa is growing up and says how nice of a woman she is becoming. Seth still sees the girl who dismissed him after he let out the truth about Captain Oats. But Marissa is growing more popular, coming home late. Jimmy and Julie don't say anything, but they are wondering what she is up to. Marissa claims nothing is wrong and if they could just leave her alone, she would be fine. She wants to be little again, with nothing going on. She wonders if this is how Seth is feeling.
When Seth is sixteen, his dad brings home a guy named Ryan, someone he's handling a case about. Ryan is cool, in that sense of being adult and mature, not cool as in popular. Seth knows this for sure when they go to Holly's beach party. Yet Seth is strangely intrigued by the fact he and Marissa seem to be hitting it off. Later on, when Seth introduces the prospect of a model home for Ryan to stay in, he gets to know Marissa, little by little. This is the girl he's lived next door for almost all his life, and yet only now is he just getting to know her. He knows that Ryan likes her and he's happy, but he still can't help but wonder what if.