The book starts off with Matthew, the main character, saying that he's going to write this letter in order to tell his younger sister Emmy about everything that happened in their family. The letter begins with the first moment that Matthew saw Murdoch, a man at the grocery store who stood up for a kid who was being abused by his father. Matthew and his other sister, Callie, are both struck by this simple gesture because they live with an abusive and unpredictable mother, Nikki.
When they get home, Matthew starts looking for Murdoch. Eventually, Callie discovers his address, but Nikki snatches it away and goes to ask Murdoch out on a date. He agrees and the kids start seeing lots of Murdoch around.
Although Murdoch is an awesome surrogate dad—he watches movies with the kids and takes them on picnics—his relationship with Nikki starts to suffer when he realizes that she's unstable. One day, Nikki drops off the kids for a long weekend without warning, and Murdoch has to look after them for four days while she goes off to party. He breaks up with her, and the kids watch as Nikki goes off the deep end. Uh oh…
One night, Nikki swerves their car into oncoming traffic until the kids all scream that they love her. She also starts to pick on Emmy—the youngest child—because she doesn't stop talking about how much she misses Murdoch. Nikki sees this as a personal attack on her parenting skills (which are pretty subpar). Matthew approaches Ben, who is his and Callie's father, asking if he'll take them in because they're not safe. Ben says that it's not possible, though, and that he can't even get custody of Emmy anyway because she's not his kid.
Nikki eventually starts dating another dude, and convinces him to go beat up Murdoch. He smashes Murdoch's car windows but doesn't manage to inflict much damage on Murdoch himself. Phew.
Afterward, though, Nikki gets her new dude to beat her up so that she can claim that Murdoch attacked her. Matthew was with Murdoch that day, however, and so he goes and provides an alibi at the police station with his Aunt Bobbie. Aunt Bobbie, who is Nikki's sister, begins to take an interest in the kids and tries to protect them. She teams up with Ben and Murdoch to start figuring out a way to ensure that Nikki loses custody of the kids, since they are all worried about their safety.
Murdoch gets a restraining order on Nikki, but she continues to harass him. When she sees that he's spending a lot of time with his neighbor, Julie, she assumes that she's his new girlfriend and freaks out—so she runs Julie off the road, getting her into a car accident that causes Julie to become paralyzed for life. Nikki has to go to jail for a few weeks.
She finally loses custody of the kids, and Aunt Bobbie and Ben step in to claim joint custody. Ben finds a nice apartment in a nearby city and moves into the new place with Callie and Emmy. Matthew decides to stay behind to look out for Aunt Bobbie when Nikki gets out of jail, since she'll probably be really mad and want to wreak havoc on everyone who's wronged her.
Nikki eventually gets out of jail and makes the house pretty much unlivable; she's constantly screaming and throwing parties and writing obscenities on the wall. It's pretty terrible.
And then Nikki figures out where Emmy is going to school and disappears with her one day. They put out an Amber Alert and a warrant for Nikki's arrest, but there is no sign of her—or Emmy—for days. Three days after Emmy disappears, Matthew gets a phone call in the middle of the night. Emmy is crying and tells him to come and get her from the Port of Boston, where Nikki has taken her. When Matthew gets there, he finds Emmy alone in a trailer and completely sloshed. Apparently, Nikki's been feeding her little daughter lots of alcohol.
Matthew starts to leave on foot with Emmy, but is stopped by Nikki on the way out. He is so angry that he throws her to the ground and is determined to kill her, but at that moment Murdoch shows up and stops him. He takes the kids home and advises Nikki to run away and never return, since she'll have to go to jail for a long time on those kidnapping charges.
Two years later, Matthew has gotten into college and is planning on leaving home in just a few months. Callie has decided to continue living with Ben, while Matthew and Emmy both live with Aunt Bobbie—who is now their new mother. Murdoch takes Matthew out for dinner one night and reveals to him that he, too, came from an abusive household, and that he killed his own father when he was thirteen. He didn't want Matthew to do the same thing to Nikki and then have to live with it.
At the very end, Matthew ends his letter and says that he's never going to give it to Emmy. After writing it all out, he realized that it's just a way for him to work through his own issues… and to finally move on.