Pauline (AnnaLynne McCord) wishes that reconnecting with her controlling mother was as easy as picking scabs, or that fixing her sisters lung disorder was as simple as dissecting road kill. Little does her mother (Traci Lords) know the extremes that Pauline is willing to go to in order to earn her approval. Maybe the other girls at school would be nicer to her if they knew she fantasized about performing surgery on them... Her mother goes to school to talk to the counsellor (Marlee Matlin), a deaf lady who despises both mother and daughter. There, Pauline learns that her mother is planning on sending her to charm school to teach her good manners. She is so trouble, that she sees a psychologist (John Waters), but then we learn that the man is not really a psychologist.
The film depicts the life of a disturbed and delusional high school student, Pauline, who has aspirations of a career in medicine and that of her younger sister, Grace (Ariel Winter), who suffers from cystic fibrosis. Pauline has vivid dreams about herself and others being mutilated with excessive amounts of blood. After each dream she awakens panting reeling from the experience in an almost orgasmic state. She decides that she wants to loose her virginity to Adam (Jeremy Sumpter); a handsome popular young man at her high school. She approaches him and explains to him what she wants gives him her number in front of his giggling friends. Like any other adolescent male at their pubescent peak he decides to take her up on the offer. Because of her fascination with blood she arranges their meeting while she is on her period. During the climax of their encounter she imagines herself choking Adam and the bed filling with blood. At the end she asks Adam to go down on her, which he obliges. After a few moments something doesn't seem quite right and he runs to the bathroom where he discovers his mouth is covered with blood. He goes to the bathroom to clean himself off, and then takes her back home in disgust. He can't get far away from her fast enough; he even tries to hide his face with a hoodie.
Back in school during sex education class, Pauline takes a sample of her blood and puts it under a microscope to check for STDs. Later on, while at school, she sees Adam on the bleachers with his girlfriend, Molly McCook (Natalie). She tells Adam she doesn't have any STDs and asks his girlfriend if she has any, giving her an indication of Adam's infidelity. Pauline then seals the deal by telling them that if the girlfriend doesn't have anything neither will she. Principal Campbell (Ray Wise) tries to crack a joke when he comes across Pauline in the school corridors.
Across the street from Pauline and Grace is a teenage girl that jumps rope in her drive way. Pauline tells her that she also enjoys jumping rope and invites the girl to jump rope with her. The girl finds the entire conversation along with Pauline extremely weird and refuses telling her as much. Grace defends her sister and storms off, pulling Pauline with her. Through out the film Pauline and the jump rope girl have uncomfortable interactions mostly brought on by Pauline. During one interaction the girl sees Pauline stop and pick up a dead bird off the street and put it in her backpack. Pauline takes the bird home and attempts to dissect it which is interrupted by her mother checking on her from outside her bedroom door. During the dissection she licks some of the birds blood off her hand and afterwords she flushes the remains down her toilet. During dinner her father (Rogert Bart) explains that the toilet is stopped up, to which she answers "not enough fiber".
Adam's now ex-girlfriend and a friend of hers TP and spray paint obscenities on Pauline's house. Upset by this, Pauline goes on an angry tirade at school in which she pushes Adam to the ground and slams his ex-girlfriend's face into her own locker resulting in her being expelled from school: Principal Campbell has no more patience for Pauline. While in her room Pauline overhears her parents arguing over her behavior and discussing Grace's doctor who wants to put Grace on the lung transplant list. During dinner that night Grace has a severe coughing fit which causes her parents extreme concern and greatly disturbs Pauline. In the morning their mother leaves, presumably for work, and the father stays home. Pauline drugs, ties up her father, and gags him. She then lures the jump roping girl to her backyard under the false premise that there are several jump ropes she can have that Grace can no longer use because of her lungs. She places a cloth over her mouth and after a brief struggle the girl loses consciousness. Pauline talks to Grace and lovingly strokes her hair, stating that Grace may be confused by what is going to happen but she has only her best interest in mind. She drugs Grace with the same cloth, then cuts and shaves her own hair. In the garage, Pauline has cut open both girls and is in the process of moving the healthy girl's lungs into her sister and placing the other lungs on ice. Pauline's mother arrives home and sees her husband tied up. She panics and runs through the house frantically screaming for Grace. She opens the garage door and finds Pauline with the bodies after she has just finished sewing them up. Pauline explains that it is her first surgery, and although it's messy, it's wonderful, and that she was unsure what to do with the jump roping girl's body so she used her to practice her incisions. She then urges her mother to take a closer look, who runs towards Pauline and grabs her, holding her and screaming hysterically. Pauline at first seems proud, but then begins to cry as if seemingly realizing what she has done...