Michael "Mike" Aaron Franks was an NIS/NCIS Supervisory Special Agent who was the leader of the NCIS Major Case Response Team and was also directly responsible for helping Gunnery Sergeant Leroy Jethro Gibbs become an NCIS Special Agent.
Born on November 22, 1948 to an unnamed father and mother in an unknown State, presumably either Louisiana or Texas, Franks subsequently enlisted in the United States Marine Corps, fighting in the Vietnam War.
He was then presumably honorably discharged and was in a relationship with an unnamed woman with the two separating after only six months. Unknown to Franks, she would give birth to a son named Liam Michael O'Neill.
After his time in the Marines, Mike joined NIS, presumably during the 80s or late 70s, and rose through the ranks of the agency, eventually becoming an NIS Special Agent as well as the leader of the main Major Case Response Team. While there, NIS was reorganized as NCIS.
In 1991 or 1992, Franks met Marine Gunnery Sergeant Leroy Jethro Gibbs as a result of Franks on going investigation into the murder of Gibbs's first wife, Shannon, and their young daughter, Kelly. Franks was also successful in enabling Gibbs to exact his revenge on the drug dealer responsible: Pedro Hernandez.
After that, Gibbs was discharged from the Marine Corps and joined NIS, becoming a Probationary Agent with Franks teaching Gibbs everything he knew about being an NIS/NCIS agent, including the headslap.
In 1996, following disagreements arising from the Khobar Towers bombing, Franks, no longer able to deal with the political fallout that was making his job difficult resigned from NCIS for good. He warned the brass that Osama bin Laden was behind the attack, but no one listened to him, so he quit. Before he left, he gave his badge to Gibbs who after Mike's resignation became the new leader of the team. He left saying "Semper Fi".
Following his departure, Franks headed to Mexico and built a small house where he would spend his days smoking cigarettes, drinking alcohol and fishing, essentially cutting himself off from the modern world.
NCIS Season 3
NCIS Season 4
NCIS Season 5
NCIS Season 6
NCIS Season 7
NCIS Season 8
For a more detailed list, see Michael Franks/List of Appearances.