The CEO of a Biotech company is killed by a car bomb and due to his work with on a ricin vaccine NCIS is called in to investigate. Deeks returns to work, but at first no one is sure if he's ready or not. The bombing is tied to an organization Omni that was involved in stealing white phosphorus which was a major component of the bomb. They also invested in Norris Biotech. In the end, it was an inside job. Someone made it appear like Omni was responsible.
Deeks struggled with his return, but in the end he was ready and everyone accepted his return.