Isaak Sidorov was a Russian arms dealer who also serves as the primary antagonist of NCIS: Los Angeles in Season 4. He has some history with Michelle Hanna aka "Quinn", the wife of NCIS Special Agent Sam Hanna.
Sidorov begins planning to steal nuclear weapons from the United States that were placed there during an operation in the 1970s.
To maintain secrecy, Sidorov sends his men after a Russian sleeper agent, James Pierce to prevent James from telling anyone including any federal agencies about his pilot while Sidorov and his men attempt to find the other warheads. Sidorov later sends one of his men after James's son Robert Pierce but after learning that the authorities are getting close, Sidorov sends his men to kill James, only for the men in question to die at the hands of Sam and Sam's partner, G. Callen. Despite this setback, Sidorov is ultimately successful in his plan to gain the stolen nukes although he and Dmitri Greshnev are forced to flee the country after their covers are blown.
Later he returns to the US looking to sell the nuclear weapons and ends up running into Quinn, with whom he has some history with, who is unbeknownst to him undercover agent Michelle Hanna. Eventually, he sets up his meeting, which is ultimately foiled by NCIS Agents Hanna and Callen but escapes due to one his men, Dmitri Greshnev, drawing away their attention and dying.
He then wishes to get back in the business with his stolen nukes.
In the Season 4 finale episode, Episode:Descent, he is responsible for placing a nuclear bomb in Mexico. He is forced to work with an undercover Sam and Quinn, who came back looking for the money he owed them and end up as replacement protection for Sidorov. He manages to set up a meeting for his nuclear devices between Naseem Vaziri and Marcel Janvier, aka the Chameleon, who tips off that that Sam is a federal agent, resulting in Sidorov and his men capturing Sam along with NCIS/LAPD Liaison Officer, Detective Marty Deeks; Sidorov and his men tortures Deeks as Sam helplessly looks on in order to find out if Quinn is an NCIS agent.
He is confronted by Quinn, who tells him that she killed one of the two girls he sent after her, after she learned his location. The two then agree to work together
He, his henchmen and Quinn then arrive at the location, he took Sam, and Deeks and has Quinn kill them to prove her loyalty, she does this however he remained unaware that Sam and Deeks were not killed.
He, his henchmen and Quinn then deliver golden bonds to Agent Vaziri and then proceed to make a deal with the nukes, Unaware NCIS was watching them.
He, his men and Quinn then travel to the Cambridge hotel, where Quinn secretly texts NCIS vital information about the nukes location, and is almost found out by him. After giving her a kiss, Rather then leave the hotel he tricks NCIS into believing they have left the hotel, while they make a secret escape, however NCIS discovers this and he, and his men are confronted on a helipad, where he takes Quinn hostage shortly afterwards, He dies on the rooftop helipad of the Cambridge Hotel when Sam shots him several times in the Season 5 Premiere. Before his death he engaged in a standoff between him and his henchman and Kensi and Sam. He threatens to kill Quinn. During the standoff Sidarov tells Sam that his love for Quinn was his weakness, to which Sam says that Sidorov's love for Quinn is his weakness. NCIS is able to retrieve the stolen nukes on board a boat.