Parker’s delinquent past resurfaces when the NCIS team is called to Philadelphia to investigate the shooting death of Petty Officer Second Class Danna Calley (Val Garrahan), and the chief suspect in her murder turns out to be Sean Doyle, the nephew of the man that Parker went to juvey with: Billy Doyle (Kevin Chapman). When the team finds out—and takes bets on what crime Parker went to juvey for [BTW, Knight (Katrina Law) wins that bet]—he comes clean and shares that the months he spent in lockup scared him straight. He added that he has managed to stay friends with Billy, even though it wasn’t easy until Billy cleaned up his act and now owns a grocery store in Philadelphia. Of course, Parker goes to get the scuttlebutt from Billy, who tells Parker that he tried to help his nephew clean up his act, but he caught him stealing from him and they had a fight. It was the last time he saw Sean. But when Sean turns up dead, it is too much, and Billy wants to take the law into his own hands, but Parker cools him down and tells him to let him do his job. Then Parker discovers that Sean was working for the DEA after getting arrested for possession, and that expands the number of suspects because the DEA had him infiltrating a bank robbery crew run by a tattooed woman named Kat Hanna (Ashley Platz), who is pretty much a ghost. Understanding that time is of the essence, Parker enlists Billy’s help in tracking her down. Billy comes through, but when Kat hears she is being charged with murder, she admits she and her crew were committing a robbery in Baltimore. It checks out, so while she may be heading to jail for that crime, the team is still no closer to finding the murderer. Meantime, Kasie (Diona Reasonover) has discovered that the gun that murdered both Danna and Sean is owned by Billy, which sets Parker off to the point where he kicks his desk and puts three shoe-sized dents in it. The last thing that Parker wants to do is have to arrest Billy for murder, because he feels that Billy saved his life in juvey. When the team can’t locate Billy, Parker knows where to find him: St. Mary’s Catholic Church, where they used to hide out as kids. But Billy isn’t alone. He is beating the crap out of Colin (Braxton Hale), his employee, who stole his gun and used it to frame him for the murders. Billy wants to shoot Colin and won’t listen when Parker tries to get him to let the law handle Colin. Finally, Parker offers to shoot Colin himself, because he can claim that Colin went after his gun. But Billy realizes he couldn’t live with himself if Parker had a murder on his conscience. So, he lets Parker arrest Colin. Then the two old friends sit on the steps of St. Mary’s where Parker admits that he really wouldn’t have shot Colin, but he will shoot Billy if he ever takes the law into his own hands. And their friendship continues. NCIS airs Monday nights at 9 p.m. ET/PT on CBS. Next, Where’s Gibbs? Everything We Know So Far About NCIS Season 19