Detective Rollins gets a visit in the squad room when her sister, Kim, shows up unexpectedly. It's clear that their relationship is strained, and Rollins excuses herself to take her sister to her apartment. She tells Benson and Fin that it's complicated.
Two young sisters, Taylor and Emily, are visiting New York on a class field trip. As they wait in the hotel lobby, the older sister, Taylor, says that she's looking for a fun party to satisfy her wild side. Wendi, a passerby, tells Taylor about a frat party that's happening later that night. She sends Taylor an email invite. Taylor's younger sister, Emily, seems reluctant, but she knows she needs to stick with her sister.
Taylor and Emily arrive at a crazy college party later that night. Taylor fits right in, and even though she's only 15, the college boys are all over her. Her sister Emily feels out of place. A college guy tries to pressure Emily into taking a shot. She is rescued by an older frat alumnus named Peter, and the two hang out for a while drinking soda and talking about sports.
Later that night, Emily searches for her sister, Taylor, who disappeared into a bedroom earlier with a drunk frat boy. Taylor is nowhere to be found, and Emily begins to feel strange. The next thing she knows, Peter is escorting Emily to the trunk of his car, telling her she'll be okay. She's dizzy and groggy, and eventually Emily blacks out.
The next morning, Taylor and Emily's teacher reports them missing. She shows Benson and Amaro a photo that Taylor tweeted, which features her in bed, naked, with a college boy. The teacher explains that Taylor is a wild child, but Emily is just a normal 14-year-old girl. The detectives discover that Taylor told another student about the party, so they head to the frat house.
The detectives locate Taylor, still in the bedroom of the frat house. Fin promptly arrests the college boy for statutory rape, explaining that Taylor is only 15 years old. Taylor doesn't know where her sister is, but Emily texted a blurry video to her sister as she was being abducted. It's not enough to ID the suspect with, but at least they know she was taken against her will.
Back at the station, the detectives analyze Taylor's phone and discover that it's been infected with malware. Taylor realizes that Wendi, the girl in the hotel lobby, sent her a party invite that infected her computer, which means that the kidnapper has an accomplice.
Meanwhile, Fin and Amaro grill the frat boy in an interrogation room. Fin explains to the suspect that he's already in serious trouble, so it's in his best interest to help the detectives as much as he can. Eventually, the frat boy gives the detectives the name of the older guy who was hanging out with Emily at the party: Peter.
Rollins runs Emily's photo through a computer database in search of a facial match online. The database pulls up a match: a live video feed on a child pornography website. Within a few clicks, the detectives are staring at a live video feed of a barely-conscious Emily dressed like a doll, waking up in a childishly decorated bedroom. Peter speaks to her from off-camera, so as not to be seen.
While the detectives discuss Emily's whereabouts in the bullpen, Taylor and Emily's parents enter the station. They quickly get in a heated argument as to whose fault this is, and the detectives are forced to separate the bickering parents. They discover that Emily's family has some issues at home, but it's nothing too serious. It's still clear that Emily was taken against her will.
Rollins decides to check in with a federal investigator in charge of tracking and eliminating child pornography rings. The analyst can't help her, but he gives her the name of a brilliant computer hacker who was sentenced to prison time for hacking into numerous websites, including shutting down several child pornography websites. This may be just the unlikely expert Rollins was looking for.
Rollins returns home to check on her sister, only to find her snorting cocaine in the kitchen with a shady-looking stranger. She kicks the stranger out, pours the drugs down the sink and lectures her sister about being a responsible adult. Unfortunately, she has to go back to work.
In an interrogation room, the computer hacker easily breaks into Peter's porn site and pulls up a directory of all his activity. With his entire web history at their disposal, Peter's pattern emerges: he kidnaps an under-aged girl, forces her to fulfill requests from users on his website, then eventually forces her into a life of crime or prostitution. The detectives decide to track down some of Peter's past victims.
Rollins and Fin set out in search of Peter's past victims. Their first target is a stripper who has no interest in talking about Peter. The second is deceased from a drug overdose. The third, a photographer with an overbearing husband, leads the detectives to Peter's accomplice Wendi, who acts as a recruiter for Peter.
Rollins get a phone call that her sister has been arrested. She heads to the scene, and a patrol officer informs her that Kim was found stumbling down the street with an opened container. The officer agrees to release Kim to Rollins; but it's clear that Kim hasn't changed her old ways, and her behavior will lead to trouble if it goes unchecked.
Benson and Amaro follow Wendi to a house in the suburbs. They promptly arrest her, and Amaro enters the house to rescue Emily. Unfortunately, the house is empty. Wendi has led the detectives to a decoy house. Just as Amaro is about to leave, a video chat screen pops up, featuring Peter and Emily in another room somewhere. Peter taunts Amaro while Benson sneaks out of the room, hoping to get a trace on the chat.
Unfortunately, the signal is impossible to trace, so Benson decides to go the human route. She contacts Wendi's real parents via video chat and puts Wendi in front of the camera. This is the first time Wendi has seen her family in years, and Peter told her that her family didn't even report her as missing. The truth comes out; Wendi realizes that her family has loved her all along, and she gives up Peter's location.
With Wendi's help, the squad locates Peter's house, and Fin bursts through the door just in time to save Emily. Peter offers no resistance, and Fin makes sure he gets what he deserves after the video camera is turned off. The detectives return Emily to her family, and all is well.
Rollins returns home to find her sister and her sister's abusive boyfriend getting frisky on the couch. Out of patience and tired of her sister's antics, Rollins pulls her gun on Kim's boyfriend and orders him out of her house. Kim gets defensive, but Rollins assures her that it's for her sister's own good. It's been a tough day.