Secret Service agent turned private investigator/attorney, Sean King was dismissed from the service after failing to protect presidential candidate Clyde Ritter due to the sun's glare and flashes from cameras.
The assassination took place years before the show is set, but King is still affected by what happens. After a alcoholic period and a stint in law school, Sean makes a break in the Ritter assassination, discovering that his fellow SS agent Bob Scott, was the man responsible for the kill.
It should be noted that though King is rarely armed, he's extremely quick-witted and can talk himself out of any questionable situation. Witnesses and even potential suspects have commented on his "charming" attitude.