First, it created a whole new genre: If you like "Buddy" movies, with two equal stars who both carry the weight of the story, and whose rapport means as much to the show as the plots, it all starts here.
It revolutionized "Location" filming, with the cast and crew traveling to Hong Kong, Japan, Greece, Spain, and Mexico to shoot scenes that would later be matched with Hollywood-filmed, studio-bound interiors, really giving an authentic international flavor to the stories.
But, and the importance of this cannot be overstated, it gave us, for the first time on American TV, a black hero who held equal billing with a white co-star, two BFFs who never even noticed that one of them was black to the other's white.
Robert Culp was an established actor and writer, and something of a star already, when I Spy started. Casting for his black partner saw a popular stand-up comedian, who had never acted before, read for the role. He was terrible. But Culp saw something that the producers had not. "Did you see that?" he asked them. "Yeah. We saw bad acting." "No," said Culp, "that was the angriest man I have ever seen! We can teach him to act, but that, that is a gift nobody can learn!" So Bill Cosby got his first acting role. He would win an Emmy for Best Actor in a Dramatic Series that year and the next year and the year after that. Nobody cheered louder for him than the actor he beat out for those awards: Robert Culp.
But the awards were in the future, on the night of September 15th, 1965, when a lucky audience tuned in, and saw that very first episode of I Spy, "So Long, Patrick Henry."
The first five episodes, including this one, are available internationally here:
The episode, by the way, was written by Culp. An amazingly talented and generous man.