When Callen's cover is blown, he's forced to cut all ties with NCIS. The man behind exposing Callen is a mysterious information broker, Eugene Keelson, who claims he has collected data on Callen that will reveal his past. Keelson's willing to trade that information if Callen performs one favor for him: deliver a flash drive to a group of Bulgarians. Callen does the delivery but the Bulgarians, believing he's Keelson, try to kill him. Meanwhile, Hetty has Eric shut down all of NCIS's electronics as Keelson has sent a Trojan horse infecting all of Callen's files. Torn between learning more about his hidden background and his responsibilities as an NCIS agent, Callen knows he can't work the case. The flash drive has information about a hit the Bulgarians are meant to perform, and while Sam and the team work to prevent the hit, Eric discovers the entire NCIS operation has been infiltrated by Keelson. This forces Hetty to bring Callen back in. Callen sets up a meet with Keelson to buy back all of NCIS' sensitive information. At the meet, Keelson sends one of the thwarted Bulgarians, knowing either Callen or the Bulgarian will be killed. Callen survives, finds Keelson, but ends up killing him in self-defense, thereby losing his connection to the one person who knows about Callen's past...Until, Eric realizes there's a house where Keelson may have stored this information. Story continues in season finale, "Callen, G."