Maggie's younger sister Erin is back in town. She's been clean for almost a year. She called because her friend Nikki's boyfriend has disappeared. Maggie and OA are called to the scene of a murder of a drug dealer, Rafael Morales, in a federal park. Morales had been in an argument with Alex Lawson. Tiffany chases Lawson, who throws a handgun before she catches him. Lawson gives up the name of Morales' partner in crime, J.J. Gomez. Maggie finds Erin injured at Gomez's apartment. Erin says two men in masks grabbed Nikki, J.J.'s girlfriend. Nikki's car is missing. Elise tracks it. J.J.visits Nikki's parents. Maggie and OA find Nikki's father tied up. J.J. falls from the second floor while trying to escape. Someone abducted Nikki to get a $35,000 debt owed by J.J. Despite Maggie's misgivings, Erin goes undercover at a club where Morales worked. Erin asks the bartender Clive where she can get cocaine. Clive's boss Sammy offers her a line and she tries to beg off. He's insistent so Erin secretly brushes off the line. But Maggie thinks she did the line for real. J.J. dies in surgery. Isobel sends Erin back into the club. Erin informs Sammy that J.J. died and tries to get him to confess. She offers him $35,000 for Nikki. Maggie wants to go in because Erin is off script. Sammy agrees to meet later and bring Nikki. Maggie insists on delivering the money instead of Erin. Erin shows up anyway at the meeting site. Maggie gives her an earwig and a locker key. Sammy, who doesn't bring Nikki, flashes a gun. Erin insists on proof of life before she opens the locker. Sammy spots Maggie and hits Erin, then Maggie shoots him. Clive has Nikki and he's arrested. Erin was clean.