While Jack Webb's second Dragnet ended in 1970, Adam-12 was rolling into its 4th season in 1972. ABC and producer Aaron Spelling decided they wanted a piece of the action. They wanted a show that was their answer to Adam-12.
The end result was The Rookies, which bowed with a TV-movie pilot on the ABC Movie of the Week in March, and graduated to series six months later. The Rookies would last for four seasons, and spawn a spinoff, S.W.A.T., thanks to a backdoor pilot in season 3.
Though filmed in LA, Spelling opted to keep the real setting of the series vague, hence making his cops members of the (reputedly) Southern California Police Department (SCPD). I think this might've been to avoid any conflicts with Webb and his family of series, and Webb's association with the LAPD.
The series followed a trio of cops, played by Sam Melville, Michael Ontkean, & Georg Stanford Brown. Mike Danko (Melville) was the only one of the trio who was married (wife was played by Kate Jackson). The Rookies served as a suitable successor to The Mod Squad, which, I think, was either at the end of its run, or would soon get there, I can't remember at the moment.
After 2 seasons, Ontkean left and was replaced by Bruce Fairbairn for the rest of the series, playing another character. After the show ended, Brown gradually walked away from acting and was married to actress Tyne Daly for a number of years. Of course, Kate Jackson segued right into another hit series, Charlie's Angels, which replaced Rookies on the schedule.
Captsisko51 uploaded the familiar, kickin' theme, composed by Elmer Bernstein, followed by a short scene in which Officer Gillis (Ontkean) is made to look like a complete dunce.