ATF agents are in a firefight with German arms dealers led by Lukas Meyer. Callen and Sam answer their call for help. Seven agents, including Callen's friend Reyes, are killed. The girlfriend of Kensi's stalker, Kessler, is found dead, and they suspect where he is living. Meyer's girlfriend, Mia Hahn, is in rehab. Fatima and Kensi are going undercover at the rehab center. The suspected buyer is Retired Army General Richard Collins, a right-wing militia leader. He and Sam, since they are ex-military, are going to meet with him. The Arizona governor called to ask them to handle Collins carefully since he's about to run for the Senate. Collins admits that he was going to buy weapons from Meyer for the militia to patrol the southern border. Sam and Kilbride get into it over how to handle Collins. Roundtree investigates Kilbride's friendship with Collins for Sam and Callen. Deeks gets briefed and trails Collins, who meets with Devon Walker, a militia leader. Fatima gets rough and takes a bottle from Mia but Kensi rescues Mia, to earn her trust. Mia tells Kensi about her abusive life with Lukas. Kensi offers to kill Lukas for Mia. Sam works around Kilbride, who finds out and cancels the warrant that Sam acquired on Collins' phone. Kensi and Mia break out of rehab. Collins tells Roundtree to tap Walker's phone instead. Mia is more concerned about her oxy fix than giving up Lukas's address. She gives Kensi the address then leaves with her dealer. Sam and Callen determine that Collins had the guns this whole time. They raid a warehouse and recover the guns, with Kilbride arresting Collins. Mia gave Kensi a fake address.  Meyer is found dead of an oxy overdose. It looks like Mia on a security feed. Kilbride is taking over OSP.