Stewart Sullivan is the field leader of 'Department S', a special Interpol unit assigned to solve especially bizarre or seemingly 'unsolvable' cases. The only American on the team and a former FBI agent, he is logical and pragmatic, and simply insists that there has to be an explanation, and will stop at nothing until he finds it, undeterred by threats or outright attempts on his life.
Stewart is a hands-on man of action and a good fighter. While his fists are usually enough to get the job done, he also regularly carries a gun and is prepared to use it if he has to. He has a subtle sort of romance going with computer researcher Annabelle Hurst, plus the difficult job of keeping the flamboyant Jason King in line and focused on the cases at hand. He also has a passion for golf (see episode "Handicap Dead"), and his favorite drink is "Lager, very cold".
