A woman is found passed out and raped at a party. At the hospital the nurse gives Fin the rape kit exams and tells him that his victim is a heavy drug user. The investigation takes a turn when the police officers, looking for the victim's ID in her purse, find a human finger inside it. Fin talks to Claire about her rape, but she doesn't remember anything. He asks about the finger in her purse and she claims she found it in the street the night before.
Dr. Warner analyzes the finger and is able to tell it is from a small child, and there are no signs of it being on the street, since there is no dirt in it. Elliot and Olivia go search Claire's home with a warrant. She lives with her Grandmother who tells the detective Claire has a turbulent relationship with her mother. After that the detectives go visit Claire's mother, she doesn't offer much help, but is not impressed to know Claire was raped or that she was carrying a human finger in her purse.
Fin gets the sketch of the man that raped Claire and is able to track him down and arrest him. At the station, Olivia tells everyone that the DNA from the finger matches the DNA from Claire, so the child is her blood relative. Elliot and Olivia interrogate Claire about it and she tells them the finger belonged to her little sister. She claims her mother killed her and put the body in a trunk inside the closet.
Olivia and Elliot talk to Claire's mother, Grace, and ask for permission to search her house for the body, but she refuses, even when the detectives tell her it will clear her name. Later the same day, the Captain informs the guys that Grace has agreed to let the police search her house the next day when she gets back from work. They find it suspicious and go watch her place to see if she'll try to get rid of any evidence she might be hiding. Fin is skeptic about it and when he is ready to give up, Grace comes out of her building carrying a trash bag and puts it in the dumpster. They arrest her and call forensics.
Grace claims it was Claire who killed the girl and she hid the body to protect Claire. At the lab, Dr. Warner examines the remains and concludes the girl was stomped to death. Grace tells Olivia and Elliot that Claire was violent and killed Lisa because she was jealous of her.
At Grace's arraignment the judge orders ADA Cabot to Charge Claire as well, and when Alex says she won't file charges against a person she thinks is innocent, the judge holds her in contempt. Ate the cell Alex's boss gives her a file corroborating Grace's testimony that Claire was a violent child that had rage attacks. Fin is forced to arrest Claire.
Claire's Grandmother posts her bail, and when she is late for court, Fin frantically searches for her. He finds her in a cheap motel, overdosing on heroin and calls an ambulance. Ate the hospital Fin presses her for the truth, she says she didn't kill her, but she feels guilty for not being able to protect her. Fin talks to Alex about it and she agrees that Claire isn't the one who killed Lisa, but with her priors, the jury will probably convict her.
Fin and Alex decide to talk to the Grandmother again. Elliot and Olivia question her about Grace's maternal skills. They decide to investigate the death of Grace's first child, a baby boy that died a sudden death when two months old. At the Rikers Penitentiary they tell Grace that they exhumed the baby's body. When confronted by her own mother, Grace confesses to killing her son and her younger daughter. When asked by Grace's lawyer what offer she could make, Alex responds no offer, she will be seeking the maximum penalty against her. Mrs. Rinato is arrested for helping Grace cover up the murder of her son.
Fin gives Claire a picture of her sister, and tells her to finally go to a rehab clinic, she starts to cry looking at her sisters face, and thanks him.