Rosie and Villa are investigating the death of a woman. They learn she's mixed up with gun dealers so they raid them and they arrest everyone there. One of them says he's ATF. They don't take his word and he asks to see Hornstock who knows him but not fondly. When they ask him about the woman he says she's his partner. Her husband thinks he placed her in jeopardy. They learn where she hides her things when she goes undercover. and they find a cellphone and they call the number she called and it turns out to be a cop who works cold cases. He says she's interested in a murder that took place a few years ago and the victim is a girl. So could she have gotten close to the killer and did he kill her.