Che is a typical teenager, living an atypical life. His parents, David and Sally, are pacifist entrepreneurs from Australia who start new businesses in different countries every few years. Their latest venture takes them to New York City where they will be helping old friends, the McBrunights, set up a new company. But what makes Che’s life different is that he alone seems to realize his 10-year-old sister is a psychopath.
Che’s three goals in life are to spar in a boxing ring, find a girlfriend and keep Rosa from hurting other people. Rosa has always been different, and her parents sent her to counselors for help, but no one guessed the depth of Rosa’s depravity. Che has tried to tell his parents several times, but they seem convinced her lying and manipulating are a phase she will outgrow.
Rosa, however, confides many secrets to Che, and he tries to teach her what is acceptable behavior and what is not. He has even had to make her promise that she will not kill anyone or convince anyone else to kill for her. This came after he learned she had made a younger friend smother her favorite guinea pig.
Che’s 17th birthday takes place the day after they arrive in New York. Frustrated and missing his friends in Australia, he sets out to find a local boxing gym. His parents hate the sport but agree to let him pursue it as long as he does not spar.
Che releases his tensions on the bag and is entranced by an African American boxer named Sojourner, called Sid by her friends. One Sunday morning, he is thrilled when he sees her in the park. She is on her way to church, and he and his family are going to meet the McBrunights for brunch.
Rosa is drawn to the men playing chess. David calls the men hustlers, but Rosa is sure she can beat them if she were allowed to play. At brunch, Che meets the McBrunights’ children. They have twin girls about Rosa’s age whose names are Maya and Seimone. Leilani is Che’s age, and she is not impressed with his humble wardrobe or his lack of sophistication. Although Che does not like her, he is glad to see that she does not fall for Rosa’s fake charm.
The following Monday, Che and Rosa’s new tutor arrives. They will be home-schooled, at least for a few months since the school year is almost over. Rosa is a genius at math and immediately wins over their teacher. When their parents text to say they will be coming home late, Rosa convinces the tutor to stay so that Che can go to his boxing class.
When Che finally gets home, he finds Rosa is missing. She used David’s cellphone to text the tutor to tell him he could leave as they were on their way home. While their parents call the police, Che remembers how much Rosa wanted to play chess in the park. He finds her there and drags her home, where she must listen to a lecture from the police about how the city is too dangerous for her to be out alone. Rosa is unimpressed, and Che tries once again to teach her about being good.
Che walks Sojourner home from the gym the next day. She admits that she likes him but they cannot date as he does not believe in God and her faith is a very important part of her life. He agrees to visit her church the coming weekend. The following day, after an argument with his mother, Che gives into his desire to spar when the trainer asks if he would like to spar with Sojourner. Distracted by her beauty, Che does little else but defend himself. Still, the trainer is impressed by his footwork.
Che loved the experience and cannot wait to try it again, even though he knows his parents will disapprove. On Sunday, Che meets Sojourner at her church. Although the choir is amazing and everyone welcomes him, Che’s only interest is in Sojourner. He is shocked to see Rosa in church. She lies when he tries to take her home, telling people that their parents beat her. Sojourner promises to teach her about Jesus if Rosa will get her parents’ permission.
Leilani texts Che and insists that he come shopping with her. She cannot be seen with him wearing his crummy clothes. The two spend the day together, and she introduces him to her girlfriend, Veronica, and their friend, Elon. Elon is androgynous and refuses to be called anything but Elon so Che cannot tell whether Elon is female or male. Leilani opens up about her frustrations with her parents and how she distrusts Rosa, especially since Rosa and her sister Seimone are excluding Maya from their friendship.
Rosa knows that Che has been sparring, She convinces their parents to stop by the gym to see if he would like to join them for dinner with the McBrunights. Sally and David are furious that he is sparring, but do not discuss it until they are home. Sally is upset that he lied and says that she cannot trust him anymore, but they allow him to keep sparring.
Che tries to convince them something is wrong with Rosa, but his parents are more worried that his lying and love of boxing are the real antisocial behaviors. Rosa talks to Che about Sid. She is worried that he might like Sid more than her.
Rosa has been trying to make Sid like her best by going to Sunday school class. She hints that she may hurt her. She also tells him that their father has an escape kit ready so he can leave the country at any moment, and she admits that she wishes Maya and Leilani were dead. Che wonders how much she says is true.
Che confesses his fears about Rosa to Leilani, who promises to keep a closer eye on her when she visits. Che, however, lets his guard down when Seimone has a sleepover at their house because he is distracted by thoughts of his first kiss with Sojourner. He wakes in the morning to the sound of Rosa’s laughter, something she never does unless she has done something wrong.
He runs downstairs to find Seimone in anaphylactic shock, while Rosa holds the auto-injector in her hand. Che administers the medicine and calls 911. Rosa insists that Seimone wanted to have a peanut butter smoothie, even though she is allergic to peanuts. Rosa swears she was going to save her friend, but Che beat her to it. Although Seimone is going to be fine, Che confronts his father about Rosa’s behavior, and to his surprise, David seems to agree with him.
Later, Sojourner spends the night with Che, and he has his first sexual encounter. Che’s father is home in the morning and is not fazed to see Sojourner. When Sid leaves, David convinces Che that the best way to help Rosa is not to seek professional help, but to continue to try and keep her behavior contained themselves.
Che and Leilani talk about what happened, and Leilani insists they tell her parents the truth about Rosa, now that David seems to agree she has serious issues. But when the families get together, David denies that he thinks Rosa is dangerous. Even Che’s taped conversations with Rosa fail to sway the parents. Back home, Sally is angry that Che spied on his sister, not that Rosa may have tried to kill Seimone.
Several months later, Che is asked to look after Rosa and the twins as their parents are busy and the McBrunights are flying in from Tokyo. Che walks Maya to her tennis lesson, while Rosa and Seimone dart around, bumping into her. As they stand at a crosswalk, Maya suddenly loses her balance and falls into the road, getting hit by a bike and knocked unconscious.
While they wait for the ambulance, Rosa loudly asks him why Che pushed Maya. When Maya dies from a brain hemorrhage, Che is investigated for homicide. Rosa and Seimone say that he pushed her into the street. Che turns his tapes and computer journal over to his lawyer to show her the facts he has kept on Rosa.
Someone, however, has put incriminating details in his journals, pretending to be Che in a rage. The McBrunights ask Rosa, Che and Seimone to have MRIs to look for abnormalities. Seimone’s is fine, but Rosa and Che both have dark areas that are indicative of people with personality disorders. Che is crushed, although the doctor and his friends tell him that he is nothing like Rosa. A video from a store camera shows that Seimone pushed Maya. Che knows that Rosa put her up to it.
The McBrunights will not pursue charges against Rosa if she and her family will return to Australia. Che is crushed to leave Sojourner, but she breaks up with him because of his dishonesty about the depth of his sister’s problems.
Rosa and Sally go to the police. Rosa confesses to bullying Seimone into pushing Maya. But Rosa tells the police her father made her do it. She taped conversations in which her father told her the only way to stop the urges to do bad things was to do them, but make it look like an accident.
Che recalls things about David’s childhood, as well as his charming, but cold, personality and realizes that his father is even more disturbed than his sister. Knowing that the police would soon discover his guilt, David leaves the country under an assumed name. Che knows he will never contact them again. The story ends as it began, with Che on an airplane next to Rosa, trying to keep her from hurting the other passengers.