Aaron Spelling dealt exclusively with ABC during the 70's, and decided he needed a break from the cookie cutter police dramas he'd been cranking out. In 1979, he gave ABC another hit, this one borrowing from an older, forgotten series.
There are parallels between Hart To Hart and the radio-turned-TV series Mr. & Mrs. North. In each case, the title protagonists are husband & wife amateur sleuths, but that's where the similarity ends. Hart sprang initially from the pen of screenwriter-turned-novelist Sidney Sheldon (I Dream of Jeannie), but his initial pitch was for the couple to be a pair of spies. Spelling and business partner Leonard Goldberg bought the rights, and with help from writer-director Tom Mankiewicz, rebooted it into the series we know today.
Robert Wagner had been a year removed from Switch when he was cast as electronics magnate Jonathan Hart, a self-made millionaire. The story goes that Spelling had wanted Cary Grant, but Grant had long since retired, and there was the belief that Grant wouldn't have been believable as Hart. Spelling wanted Wagner's real-life wife, Natalie Wood, to play Jennifer, but Wagner was, according to accounts, "reluctant". Stefanie Powers, whose last two series, The Girl From U.N.C.L.E. & The Feather & Father Gang, each lasted a season, landed the part. Lionel Stander played Max, the chauffeur/bodyguard/cook, and also narrated the open.
Hart To Hart lasted five seasons, and fell into the predictable formula storytelling of other Spelling shows (i.e. Charlie's Angels). It took viewers a while to warm up to the show, which cracked the top 20 in its 2nd season. For Wagner, this was his first series to go beyond three seasons, as Switch & It Takes a Thief topped out at three each.
A Hart To Hart fan channel uploaded the 1st season open to YouTube. Bear in mind that the narrative changed beginning in season 2 to what most of us may be familiar with.
Spelling would go the millionaire-as-amateur-sleuth route again with Matt Houston during the 80's, and I think it was while Hart was still on the air.