Grace and Tippi are 16-year-old conjoined twins. They have two heads, two hearts, two sets of lungs and kidneys and four arms. Their intestines begin apart and then merge, and below that, they are one, sharing one pair of legs.
They live with their mother, father, 14-year-old sister nicknamed Dragon and their grandmother. Tippi and Grace have been home-schooled for their entire lives, but with financial donations running out and their father unemployed, they must start attending traditional school. The city pays for the girls to attend a private high school, Hornbeacon High.
Both Grace and Tippi are nervous about attending school, as strangers have been cruel to them. Just before school starts, the girls catch the flu. While Tippi recovers quickly, Grace has a harder time getting better.
When the twins start school at Hornbeacon, another student, Yasmeen, is assigned to be their guide and friend. Yasmeen thinks Grace and Tippi are amazing and doesn’t become sickened when she sees them. In their first class, Grace is fearful of being stared at. Yasmeen, despite her small stature, defends the twins with curses and threats to anyone who gives them odd looks.
In art class, they meet Yasmeen’s friend, Jon, who treats the twins like they are two people. Instead of going to study hall, Jon and Yasmeen take the twins to an abandoned church in the woods behind the school to smoke. By the end of the first day, Grace thinks that she and Tippi may have found friends.
While getting ready for the second day of school, Grace passes out while brushing her teeth, but she lies to Tippi and her family and tells them that she slipped. At school, they find an offensive note taped to their locker, telling them to go back to the zoo. Grace feels the reality of their situation has arrived — that the other students will never accept them.
Yasmeen tells the twins that the students are afraid of them, just like they are afraid of her. Yasmeen has HIV. As the girls spend more time with Jon and Yasmeen, Grace develops a crush on Jon.
One morning at school, Grace and Tippi overhear Jon telling Yasmeen that the twins’ situation is unfortunate and a waste because they are so pretty. Both girls are angry, and while Grace wants Jon to explain what he means, Tippi drags her away. Yasmeen and Jon apologize, but the twins stay angry with them for a while. Eventually, Grace and Tippi go back to the abandoned church to drink, smoke and hang out with their friends.
Grace becomes more out of breath while doing physical activates. She continues to lie to her family, but Tippi knows something isn’t right. While Grace eats sparingly, Tippi is the one losing weight. Additionally, the twins worry over their family’s finances.
Dragon starts teaching younger ballet students in order to pay for her own studio time, and the twins are angry that their little sister must work. Dragon’s ballet studio is also planning a six-week trip to Russia, but Dragon cannot go because all the family’s spare money is spent on the twins’ healthcare. Grace wants her sister to reconsider paid offers to appear on television, but Tippi refuses, insisting that they keep their dignity.
When their mother loses her job, the family’s financial situation becomes dire. Faced with the possibility of selling their apartment and moving, Tippi relents and agrees to let a documentary filmmaker, Caroline, follow their lives for several months. With the $50,000 they earn, the twins pay for Dragon’s dance trip to Russia, providing their sister with a way out of the invasive filming of their family’s lives.
While their dad tries to avoid the cameras, he fails, and it’s easy for everyone to see that he is drunk. After their dad has an argument with their mom, the couple decides that it’s best for him to move out until after filming is complete.
After a few weeks, the twins become accustomed to being filmed. When Tippi loses consciousness and falls on school grounds, the camera crew reacts immediately and calls the ambulance. The doctor tells the family that when the twins caught the flu, the virus severely weakened Grace’s heart, and Tippi’s heart has been strained with the task of supporting both bodies. For the girls to survive, they must be separated and Grace needs a heart transplant.
If they remain conjoined, they will both die. Grace blames herself and her heart for quietly killing her sister, so she agrees to the surgery. Their parents are terrified of losing either daughter, but they let the girls decide.
Caroline doesn’t try to film the girls in the weeks following their diagnosis, proving that she isn’t after a sensational story. When she does ask to start filming again, Tippi and Grace agree, knowing this may be documentation of the final months of their lives.
Against their mother’s wishes, the twins go with Jon and Yasmeen for a weekend away during winter break. Tippi and Grace bring alcohol from their newly sober father’s stash. At the vacation house, Jon and Yasmeen push together two beds to make a sleeping space large enough for the four of them. Yasmeen sleeps beside Tippi while Jon sleeps beside Grace. They spend the days drinking, smoking cigars and skinny-dipping in a neighbor’s pool. At night, Jon and Grace kiss.
After Christmas, the twins get ready for the separation by undergoing a painful surgery to put in skin expanders. By January, the girls are ready for the separation surgery. They say sad goodbyes to loved ones and to each other. The girls are kept unconscious for a week after the surgery to spare them much of the pain.
Grace wakes up to learn that she has done well and is on the list for a heart transplant, but Tippi lost a lot of blood and has an infection. Grace begs to see Tippi. When Grace sees how sick she is, she tells Tippi that it is OK for her to die, and Tippi does.
Grace’s grief over Tippi’s death is so intense, it causes her physical pain. Grace’s recovery is slow, and she remains in the hospital, waiting for a donor heart. Caroline interviews her and Grace acknowledges that while Tippi was always good at being heard, Grace has hidden herself from the world for too long. She has been a coward, and now it’s time to tell her story of what it’s like to be two and to be one.