The NCIS team investigate a charred corpse, who is found to be Lachlan Colston of the Royal Australian Navy, a foreign exchange officer who was working on sensitive nuclear negotiations between the U.S. and Australia. The RAN sends an investigator, Naomi Parsons, to assit the team.
The murderer stabbed Colston to death and took a finger as a trophy. This points to the death being a murder-for-hire. Patton Plame locates the hitman on the Dark Web and orders a fake hit to draw him out, but Parsons jumps the gun and the hitman gets away, leaving only the gopher, revealed to be LaSalle's informant, Ross P. They are able to ID the hitman from his vehicle and when they find his lair, they realize that Colston may not have been the intended target.
The real target may have been Lt. Ryder, the man Colston exchanged with. Ryder points them to his bar singer girlfriend Cheryl, who directs them to a bookie named Sugar Wells. Sugar denies any involvement, even thinking that Ryder was the dead man. When the team finds Colston's finger in a package intended for Ryder, they wonder whether Ryder was the one who hired the hitman. With Ross P.'s help, they find the hitman, only he's been murdered, too. On the hitman's phone, they find surveillance photos of Cheryl... with Colston.
Cheryl realizes that her piano player Luke has been jealous of all her boyfriends and wants her all to himself. The team gets his confession on tape and arrest him.