After a chaotic shootout at a gala event, the Five-0 task force investigates the death of a man who appears to be an innocent bystander. However, the investigation reveals he was actually a professional hitman. The team must track down his intended target—a mysterious "third man"—before a second assassin can finish the job.
The title refers to the unidentified third person involved in the initial shootout who becomes the focus of the manhunt.
This episode is pivotal for the "Shelburne" arc, which serves as the primary overarching mystery of Season 2.