Hetty and Granger inform Sam that his former partner, Mark Ruiz who left NCIS because of a substance abuse problem, who is now working with the ATF, has vanished. He was working on finding a new military explosive that someone is trying to sell. There was a meeting and from surveillance video, they show Sam that there was an explosion and Ruiz may have been the one who set it off. Sam doesn't believe that, but they need to find him and he agrees to do so. He sends Kensi and Deeks to check out where the meeting took place, without telling them why he left NCIS. While he and Callen go to see Ruiz's wife. She says she hasn't heard from him for weeks. She gives them his cell number and they find him. He says he didn't set off the explosion. The seller tried to pull a fast one on the buyer and he made off with the money. He thinks he can convince the guy that he found another buyer. Sam agrees to let him to do it while Callen is not sure he can be trusted.