Simon Cade was an NCIS Special Agent working in Special Agent Erica Jane Barrett's MCRT.
Cade was a senior agent, working with Special Agents Barrett and Gayne Levin. He was a genius with an IQ of 160, attended and played football at Yale University, and was fluent in several languages, as mentioned by both Abigail Sciuto and E.J. during the episode "Dead Reflection".
After E.J.'s team was reassigned to Washington, D.C, he became close with Abby, becoming good friends on her and even went on a date with her which left Abby's former boyfriend, Timothy McGee jealous. Cade was also close friends with E.J. and Gayne, and was friendly with all the members of Gibbs's team.
Cade was severely injured during the hunt for the Port-to-Port killer when the killer, Lieutenant Jonas Cobb, ambushed E.J.'s team. In the ambush, Levin was killed after being shot, and E.J. was captured, while Cade nearly died of his injuries but thanks to Anthony DiNozzo's efforts, managed to survive just long enough to be taken to hospital where surgery was performed on him. Abby even stayed by his bedside.
Following his near-death experience, Cade returned to his home in Bremerton, Washington.
Unfortunately, the newly-appointed Secretary of the Navy, Clayton Jarvis, believed Cade to be guilty of treason due to misinformation provided by the true traitor, a high-ranking Naval official named Sean Latham and assigned DiNozzo to kill or capture Cade, who had disappeared upon his return to Washington. Tony, attempting to locate Cade, found E.J. and convinced her to bring Cade back to Washington D.C, without informing her of Cade's suspected treason.
Latham, attempting to cover his tracks, ordered an operator under his command named Jonathan Cole to assassinate both Cade and E.J. Cade was immediately forced on the defensive upon his return, as Cole was hunting him. He called Abby for help, resulting in Gibbs's team also attempting to locate him.
Cole hacked E.J. and Tony's phones, sending messages to each of them meet in a small alleyway.
Once the three agents had met and while Tony was attempting to arrest Cade, Cole stuck, ambushing them. He fired with his shots leaving both Tony and EJ injured. As a result, EJ fled the scene, going into hiding while leaving an unconscious Tony on the ground although Tony was later found and brought to hospital, having suffered memory loss.
Cade wasn't so lucky as the shot that hit him killed him instantly. After that had happened, Cole fled but not before planting DiNozzo's NCIS badge on Cade's body to make it look like Tony had killed Cade but his efforts to discredit DiNozzo failed.
Months later in the episode, Housekeeping (episode), Tony and EJ later succeeded in clearing Cade's name.