Watch an example (a montage of montages, actually) here from YouTube. Each episode’s intro features the famous theme song with a montage of the upcoming episode, visually tied in with a burning wick. Each episode then had a unique intro. Another clip from the first season shows that Peter Graves wasn’t in the first season, Steven Hill (later of Law and Order) was. The fifth season features Mr. Spock, er Leonard Nimoy. I think in the early 1970s Nimoy was in his “I am not Spock” phase.
Hawaii-Five-O is arguably the best, it had a spectacular intro.