The murder of a cab driver just hours after he sent an email to an old alias of Callen's triggers NCIS: LA to reopen an old case. Four years earlier, Callen spent 6 months in deep cover as a corrupt MP: his goal was to get close to 3 Marines who stole $5 million and collect evidence to convict them, as well as recover the money. They convicted all 3 Marines, two of whom cooperated, however, NCIS didn't find the money because it had been moved and the felons didn't know where. The murdered cab driver was one of the conspirators, recently released from prison and he emailed Callen for his help because he'd found the money. Kensi interviews the one thief still in prison (he didn't cooperate) and finds herself being interested in his lawyer. Meanwhile, Callen resumes his past undercover role to track down the third conspirator, Jon Donnelly. Sam tracks Callen, worried that he's going "lone wolf" on him. Sam isn't pleased to discover Callen had a romantic relationship with Donnelly's sister and she has a 4 year old son (who turns out not to be Callen's). Through the sister, Callen finds Donnelly but he's murdered before Callen can speak to him. They realize the third man still in prison must be the connection to missing money and the murders, but when he's also killed, they figure out his lawyer stole the money and is getting rid of all the evidence.