With Charlie's impending departure date for Cambridge, he and Amita have had to move up the wedding date. The couple has Larry performing the ceremony at a close, intimate wedding. Now with Charlie married, Alan is determined to move out of the house. He said he would when Charlie got hitched.
While working on a case, Don's gun is knocked out of his hand while he's making an arrest and the gun is stolen. The FBI team is focused on locating Don's gun. LAPD contacts Don saying they have a lead on his gun. Two drug dealers were found dead in a bad part of town. The marks from the murder weapon are a match with Don's gun. The team does some research and find that there are several social networking sites that reference "Cop's gun" as well as a photo of the slain drug dealers.
There's a vigilante gun that has been taking care of things that the police haven't been able to do. This is the message being spread across these social networking sites. This gun is being passed around to different vigilantes around town covertly. The team follows this trail and it leads them to a suspect, someone who has been the victim of a terrifying neighbor. The police have not been able to arrest this neighbor.
Don's gun was picked up by that family's teen-aged son who was hoping to fix the problem. Don gets the gun back from the teenager and Ian arrives to arrest the neighbor – a fugitive from the Florida State Prison.
Larry and Alan decide to renovate the garage into a guest house where Alan can stay while Charlie and Amita are at Cambridge. David accepted an offer to run his own team in Washington D.C. Don proposes marriage to Robin and she accepts.