A nuclear bomb went off in a remote area and it's presumed to be one of the three nukes that Sidorov stole. No one claims responsibility for the detonation which further supports the theory that it was to prove the nukes were viable and raise the price.
In order find the nukes, NCIS decides that they need to find a known arms dealer to make play for them. Hetty breaks up the normal pairings and sends Callen and Kensi to Iran to find and bring Marcel Janvier back the United States to help them. And, has Sam and Deeks work together with Michelle to watch over Sidorov. Deeks is offended by Sam's continual knocking of his abilities and lack of trust.
After bringing Janvier back from Iran, Callen convinces him to contact the Iranians to see if they want to buy the nukes and then to call Sidorov to buy them. Janvier agrees to do it, but wants NCIS to give money to his daughter.
Quinn and David (Michelle and Sam Hanna) get back in good with Sidorov to protect him during the negotiations and meeting for the sale of the bombs. Deeks and Kensi cover them. The first meet seems to go well and an upfront deposit in gold is set up. After the gold is paid and verified as authentic it all goes bad. Sidorov sends Quinn with two Russian escorts, but not before he asks her to run away with him. He doubts her feelings and she kisses him. As Deeks and Kensi look on they decide not to tell Sam about that.
As Kensi leaves to follow Michelle, she complains about Deeks communication problems and Deeks kisses Kensi. She is surprised, but has to leave before they discuss it. Meanwhile, Sidorov pushes Sam in a pool attached to a 100 pound briefcase of gold. Deeks goes in to rescue Sam. After resuscitating Sam, they are both taken by Sidorov.
Callen wants to know why Janvier gave up Sam. He put "David" on one eyelid and "Agent" on the other to warn Sidorov. Janvier blames Callen for everything that happened to him including the loss of his hand. He wants Callen to pay with the death of the partner he loves, Sam. Janvier asked Callen to kill him earlier and still wants to die, but he wanted payback first.
Sidorov is convinced that Quinn is also an agent and tortures Sam to verify it. He stays strong and insists that she's not an agent. Sidorov moves on to Deeks. He claims he's a LAPD officer that was doing surveillance on the house for drugs and that's why he showed up. Sidorov tortures Deeks for information about Quinn too, but he holds strong as the episode ends.