Michelle Lee was a Special Agent who joined the NCIS team beginning in the Season Four opener, "Shalom." Leon Vance placed Lee on Jethro Gibbs' team when she returned in the Season Six opener, "Last Man Standing," because a planted mole was leaking US military info to the enemy, and Lee and fellow agents Langer and Keating were suspects. Later in the episode, Lee and Langer investigate the computer archive center while Keating is being interrogated, but Lee would give a frantic call to Gibbs labeling Langer as the mole. Gunshots were heard, and Lee is found with Langer, who was shot dead by her; Lee claimed she did so in self-defense.
In the final scene of the episode, Lee turned heel when she was revealed as the true mole, receiving a text from her contacts asking if the rest of the NCIS team suspects her, to which she replies, "No." When Langer suspected Lee as the mole, the evil Lee killed him to protect her cover, establishing Lee as a villainess. When Vance and Gibbs learn the truth in the episode, "Cloak," they set a trap to catch 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 the 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 actual 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 shot kills both Lee and Bankston.