After a distraught woman comes into the SVU demanding the detectives investigate her granddaughter's disappearance, they decide to interview Ruth's daughter, Ashley. Ashley blames her mother for interfering in her life and says she was about to pick up her baby daughter at the babysitter's house. Stabler offers to accompany Ashley. When Maria, the babysitter, is nowhere to be found at the residence, Ashley flips out and says Maria must have kidnapped Sierra, her little girl.
After the detectives are quick to find Maria and let her go for not being a suspect, new surveillance video comes in showing Ashley purchasing a shovel with a stolen credit card. Ashley swears that's not her on the videotape, but she's clearly lying. Ruth flips out and attacks Ashley, but Fin and Stabler are quick to break up the fight.
Ashley threatens to walk out of the interrogation since Stabler keeps accusing her of knowing where Sierra is. Stabler has no choice but to arrest her for credit card theft. Cabot is disappointed that Stabler couldn't pin a confession but there was no way he was going to because Ashley's father, Ralph, had contacted a lawyer to help with his daughter's case.
Ralph hired Sophie Devere to represent his daughter. She immediately uses the press to help garner sympathy for her client. Meanwhile, Stabler discovers that Ashley wasn't lying when she said she bought the shovel as a gift for her father. Ralph hands the present over to Stabler for further investigation, but the lab reports no evidence can be found that would conclude the shovel was used as a weapon, but the lab did report high amounts of the chemicals found in petroleum.
Stabler returns to the Walker residence to find Ralph drinking and watching old home videos of Sierra. Ralph cries as he talks about Sierra being gone and how much he's going to miss her. Stabler reminds Ralph that his job is to find Sierra and that's his only job. He needs Ralph to focus and join him in trying to find her. Ralph points out a popular empty lot next to a gas station where Ashley used to go party with her friends. Stabler calls in a search crew to help him start digging.
When the search dogs start digging at a spot in the lot, Stabler and the rest of the search crew begin to dig and discover a baby girl's body buried, along with her doll. Meanwhile Cabot, in court with Ashley and Sophie, gets an email on her phone about the discovery.
At the news of Sierra's body being discovered, Cabot alerts the courts that she's tacking on murder charges, as well. Cabot announces that Sierra's body has been located and Ashley goes hysterical. The Judge sets Ashley's bail at one million dollars. But, just as Cabot is leaving the courthouse, she gets more surprising news from the lab.
Warner tells Cabot that Sierra died from measles and not from homicide. Cabot agrees to drop the murder charges against Ashley, if she will cop to community service and participate in alcohol treatment and come clean about what happened to Sierra the night she died. Ashley admits to spanking Sierra and the next day waking up to find her dead and then panicking and burying her. Ashley feels guilty that she killed Sierra by spanking her, but Cabot tells Ashley the real reason she died. Sophie accuses Cabot of tricking Ashley to get her to confess to burying her child and she could have walked, but Cabot defends her choice, saying now Ashley gets a second chance at a clean life.
Warner alerts the detectives, they have a bigger problem now and that is tracking down patient zero for the measles outbreak. Once they isolate the disease to a teenager living in an Amish town, the next move is to track down children who may have come in contact with him. Fin heads to the park where the teenager met his secret girlfriend, and where Sierra used to play.
The detectives end up at Monica Stewart's house, a mother known around the neighborhood for not vaccinating her children. Monica refuses to listen to Stabler as he accuses her son of infecting Sierra with measles. When Stabler and Benson return to the SVU, they are slapped with worse news; Ashley and her family are suing the city of New York for letting their city park expose Sierra to measles. Cabot warns Stabler that the Walker family has a very strong case and orders the detectives to arrest Monica Stewart for murder.
Cabot calls Warner to the stand to discuss the alarming facts about measles. Warner's main point is that it is more dangerous to not vaccinate your child and expose them to the risk of measles, than to actually go with the decision of vaccinating your child, preventing risk for children everywhere. Warner's final statement tells the courtroom that Monica's negligence caused Sierra's death.
Cabot questions Monica on the stand. Monica continues to stand firm that she made the right decision by not vaccinating her child and says that Ashley was a terrible mother, so something bad was bound to happen. Cabot reminds Monica that she's the one on trial. At the end of the day, Cabot is sure the jury will come up with a guilty verdict, it's just a matter of time.
Back at the SVU, Stabler, Cabot and Benson wait for a call about the verdict. A debate ensues on who was the real guilty party. Cabot and Stabler put all the blame on Monica for exposing Sierra to measles, while Benson continues to Monica's point: she was telling the truth when she said she had no idea her son was still infected when she took him to the park to play with the other kids.
The verdict comes in and it is "not guilty." The Walker family is completely shocked, especially when Monica blames Ashley for the death of her daughter. A distraught Ashley runs out of the courtroom crying. Benson finds Ashley in the bathroom and tries consoling her, but is interrupted when her father comes in to talk to his daughter. Their moment is interrupted when Ruth comes in, vowing revenge against Monica for the death of her granddaughter.
It doesn't take long for Ruth to act out her revenge. The detectives get a call and rush to Monica's residence. There, they find Ruth and Ashley have thrown a brick through her window and continue to scream at Monica inside her home. Stabler tries telling Ruth to let go of her thirst for revenge. As Stabler and Benson are preoccupied with both women, Ralph rushes into Monica's house and they hear a gunshot. The detectives find Ralph's committed suicide in front of Monica while clutching a photo of him and his granddaughter. Both detectives are devastated with Ralph's tragic end.