It seems like it’s been a lifetime since director Michael Mayer first had the idea to do a major gay narrative overhaul of “On a Clear Day You Can See Forever,” the 1965 tuner about past lifetimes and reincarnation.

It was 15 years ago, he reminisced at the Plaza, where the show’s opening-night party was held Sunday. The idea came to him, all at once, in 1997 as he was working on “Side Man” for New York Stage and Film. “I was walking across the Vassar College campus singing the score, and it hit me like a ton of bricks,” he said.

It took about a decade to find the right book writer in Peter Parnell — but it certainly wasn’t the scribe’s abiding belief in reincarnation that earned him the gig. “I fall pretty firmly on the other side,” Parnell admitted. “My husband is a psychiatrist.”