Special Agent Michelle Lee joined the NCIS team beginning in the Season Four opener "Shalom", but she would be placed on Jethro Gibbs' team when she returned in the Season Six opener, "Last Man Standing." She was placed on Gibbs' team by Leon Vance because a planted mole was leaking US military infomation to the enemy, and Lee was a suspect, along with fellow agents Langer and Keating. Later in the episode, Lee and Langer investigate the computer archive center while Keating is being interrogated, but Lee later gave a frantic call to Gibbs labeling Langer as the mole. Gunshots were heard over the phone, and Lee is found with Langer, who was killed by Lee in self-defense (according to Lee herself).
Lee turned villainous when she was revealed as the actual mole in the episode's final scene, receiving a text from her contacts asking if the rest of the NCIS team suspects her, to which she replies, "No." Langer suspected Lee as the mole, and while they were in computer archive center, the evil Lee shot and killed him to protect her cover, cementing Lee's heel turn.
When Vance and Gibbs learn the truth in the episode, "Cloak," they set a trap for Lee, which succeeds. In her interrogation, Lee confesses to being the mole, but she stated that she did what she did because her daughter, Amanda, was held captive, and she was forced to commit these acts or else they would kill Amanda. It was later revealed that Amanda is actually Michelle's sister, but she had raised her as her daughter since the death of their parents.
In the episode, "Dagger," Lee worked with Gibbs to catch the real mastermind, who was revealed to be Ted Bankston. In a final confrontation, Bankston holds Lee as a human shield while Gibbs targets him. Despite this, Lee gives the signal for Gibbs to shoot, and his shots kill both Lee and Bankston.