"Identical" is written from the alternating viewpoint of the twins, Raeanne and Kaeleigh Gardella, with chapters switching between their two perspectives. Their narratives are written in free verse in sometimes jagged shapes of broken hearts or liquor bottles, with the appearance of the text reinforcing the content. Each twin's chapter ends on a left-hand page, while the other twin's chapter begins on a right-hand page. Where the two are juxtaposed, selected words from each are identical, set off from the rest, as if trying to join with the words from the other twin on the opposite page. This technique illustrates the twins' deep connection.

Raeanne and Kaeleigh, are both troubled high school students. Their family had been torn apart years before, when their Daddy drank too much and swerved their car into oncoming traffic. There was a fatality and Mom lost her ability to love. Mom is never home these days. She is running for a seat in the U.S. Congress. Daddy is a judge, stern and domineering. Daddy started looking for the love he couldn't get from Mom by visiting little Kaeleigh in the night, back when she was only ten. Young Raeanne could only watch, scared and jealous. Kaeleigh copes as best she can with Daddy's continuing sexual abuse, trying to be a good girl, finding solace in binge eating and cutting herself with a razor. Raeanne, Kaeleigh's twin, wishes Daddy loved her best and that she could pleasure him. Raeanne looks for something to fill the void inside her. She trades sex for marijuana and becomes more and more reckless. Raeanne steals OxyContin and whiskey from Daddy, and she experiments with drugs and bondage.

Kaeleigh finds escape at a Lutheran home, where she has a part-time job. One resident, Greta, reaches out to Kaeleigh, sensing her pain since she herself had been abused by her father. Kaeleigh also has a loving boyfriend, Ian, who understands that Kaeleigh is troubled but does not know how to help her. Kaeleigh wishes she could open up to Greta or Ian, but she cannot. Sometimes Raeanne comes to Kaeleigh's rescue when Kaeleigh seems unable to help herself.

A few days after Mom wins the election, Kaeleigh overdoses on whiskey and pills. Daddy finds Kaeleigh and seeks help from Hannah, a nursing-student neighbor with whom he had had an affair. Hannah helps Kaeleigh vomit and then tells Daddy that Kaeleigh needs help, but Daddy wants to handle things himself, as usual.

A desperate Raeanne goes off in search of Grandpa Gardella, Daddy's father, to find out if secrets from Daddy's past might explain why he is such a demon. Raeanne learns that her Daddy was sexually abused by a neighbor when he was young. Raeanne understands Daddy better, but she still needs to feel something, anything. Raeanne goes off with Ty, a young man who is into drugs and bondage, and just when things start to get out of control, Ian arrives to rescue Kaeleigh. Ian is disappointed that Kaeleigh is with Ty, but Raeanne explains that she is not Kaeleigh, not her twin sister. Ian, devastated and shocked, explains that Raeanne is dead, killed in a car crash years ago, and the girl standing here is indeed Kaeleigh.

A doctor helps Kaeleigh to understand that she probably has dissociative identity disorder where she has created multiple personalities to cope with the trauma in her life. Daddy is forced to go into rehab and then find an apartment thirty miles away. Mom only comes home for rare visits, for the sake of appearances. Grandma Gardella moves in with Kaeleigh, in the family home. Ian promises to stand by Kaeleigh and love her, as long as he is able, to help her through her treatment. Kaeleigh vows that the abuse stops here, that her children will be protected and nurtured. Raeanne, lurking quietly inside of Kaeleigh, vows that she will never leave, because she is strong and she must protect her twin.