A girl gets off the subway in Times Square on a rainy evening and meets her friend for a night of fun, passing by a man in a tourist t-shirt. The man wanders through the crowd before getting mugged by a man and a woman. As the two girls get into a black van, one drops a gum wrapper, which floats past the now dead body of the man. Mac and Hawkes show up at the scene and Flack informs them that the victim was found without a wallet or ID. Hawkes notices a scar on the man's neck, evidence of cataract surgery and rings on his nails, indicating he had cancer and was treated with chemotherapy. His acrylic dental work leads Hawkes to surmise that he was from the Ukraine. Back in the lab, Stella finds evidence the man spilled coffee on his pants but not his shirt. When Danny finds the remnants of a price tag in the shirt, Stella wonders if the man purchased the shirt at the airport. She asks Danny to check into incoming flights at nearby airports. Sid traces residue on the knife to flowers found in Chinatown, while Lindsay is able to get DNA off a lip piercing from the body and match it to a man named Sammy Chen. Danny and Flack go to Chinatown and chase down Sammy and his partner in crime, Jody Sun, and bring them in for questioning. The pair admits to mugging the dead man, but when they found he only had rubles on him, they tossed his wallet up on an awning. He got the knife away from them and they ran off--leaving him alive. Danny and Flack climb to the top of the awning and recover the wallet, along with an ID identifying the man as Adrik Fedoruk, a letter damaged by the rain water and a piece of paper with a meeting time and location on it. Mac goes to the Deluca Motel and finds Deputy Inspector Gillian Whitford waiting. She tells him that Fedoruk's daughter Rani had come to New York a month ago and disappeared. Fedoruk wrote the mayor and the letter ended up on Gillian's desk, and she agreed to meet with the man.
Danny and Hawkes delve into processing the letter: Hawkes translates it from Cyrillic and is able to decipher that Rani had written to her father asking him to come get her. The letter was postmarked four days ago. Danny lifts DNA from the stamps on the envelope and matches it to a club owner named Willie Burton. Stella pays Burton a visit, but he only vaguely recalls mailing the letter for the girl after she approached him at a party at his club and asked for a favor. Frustrated, Stella heads back to the lab and recovers a glove print from under the armpit of Fedoruk's sweatshirt, leading her to believe that his killer helped him up before stabbing him. Trace from the glove print is made up of multiple elements, including rust, plaster and several spices used in the making of mustard, leading Hawkes to suggest that perhaps the girls are being held in an old abandoned mustard house in Williamsburg. The team storms the warehouse, finding it filled with mattresses and blankets. Mac has a disturbing realization: their killer is a sex trafficker. Flack catches a young woman hiding under some blankets. She tells Mac and Stella that her name is Katie, but she claims she was just crashing at the warehouse. Assuming she is traumatized from her experience with the traffickers, Mac and Stella allow her to use the bathroom, but she locks the door and breaks out through the window and runs away. Gillian Whitford surveys the scene and estimates thirty girls are being held, and when Lindsay runs a hair analysis, she notes that the girls are all between the ages of 14 and 21. Lindsay also finds evidence that the girls are being drugged with a combination of heroin, ecstasy and codeine. A search of the drug database reveals this particular drug mix is unique to a dealer named Nemo. Stella floats the idea of getting Willie Burton to set Nemo up.
Flack brings Willie in and while he's reluctant, he eventually agrees to go along with the plan for the sake of the girl he mailed the letter for. Gillian is upset when she finds out he wants to do it his way--with no undercover cop involved--and Mac follows her after she storms out. She tells him why the case is so personal to her: her fourteen-year-old niece, Rachel, went missing three years ago. She relents and tells Mac he can arrange the set up however he needs to. Patrick Habis, the father of one of the missing girls, shows up at the station, frantic about finding his daughter Tara. Stella sits with him as he describes Carolyn, the girl his daughter Tara met in New York who had promised to take her to LA with her so they could pursue acting careers. A single father, Patrick tells Stella that he didn't hear from Tara after she went off with Carolyn, which wasn't like her. Willie meets Nemo and as soon as the drugs change hands, Flack and his team burst in, arresting the men. Flack whispers a quiet thanks to Willie. In the interrogation room, Flack and Danny lean on Nemo about him selling large amounts of drugs to a sex trafficking ring. Nemo gives up the name and location of the man he's been selling to, and Mac and Stella storm it along with the cops. Flashbacks reveal that Fedoruk went looking for his daughter and the sex trafficker followed him, killing him after Chen and Sun mugged him. Gillian brings Rani to view her father's body and the tearful girl tells him that he saved her and the other girls. Stella reunites Tara and her father, and Tara recognizes the computer rendition of Carolyn and identifies her. Stella realizes Carolyn is actually Katie, the girl who escaped the warehouse--she's in league with the trafficker. Katie greets another young girl fresh off the subway and prepares to get her into a black van--until the doors open, revealing Stella. Mac catches Katie before she can run off and arrests her while Stella gets the unwitting girl to safety.