Fresh from his success in '77 Sunset Strip', Efrem Zimbalist Jnr starred as the tough and incorruptible G-Man, Inspector Lew Erskine, an unrelenting pursuer of the unlawful and unrighteous, who would traverse the length and breadth of the United States tracking down communists, extortionists, counterfeiters, radicals and the godfathers of organised crime. In the early episodes Erskine had a daughter, Barbara, (his wife had died in a shoot-out) who dated fellow FBI agent Jim Rhodes. However, the producers decided to drop the character in order to concentrate the action on Erskine's dogged pursuit of justice.

The series always portrayed the F.B.I. in a favourable light leading it to win the recommendation of Hoover, who took the unprecedented step of allowing Efrem Zimbalist Jnr to spend a week with real-life agents and a day at the F.B.I. Academy in Virginia.

"My visit there," said Zimbalist Jnr, "consisted mostly of interviews with personnel in charge of the various divisions of the Bureau, ranging from counter-espionage to domestic crime, and instruction in the various technical departments such as the laboratory and ballistics." Zimbalist Jnr was also given instructions in hand-to-hand combat. "We were constantly with members of the Bureau, and the familiarisation was an ongoing process."

Further endorsement of the series was given by Hoover in allowing certain scenes to be filmed both in and around the F.B.I.'s Washington HQ. Meanwhile, Erskine and his fellow agents were always seen driving gleaming new sedans, supplied by the series official sponsors, The Ford Motor Company. Some episodes were given a further touch of realism by a short closing segment in which the real-life F.B.I. would make an appeal for information on their 'most wanted fugitives' including, in 1968, the assassin James Earl Ray.