"Yogiri no Shinobi Ai" (An Affair in The Night Fog) has turned out to be one of those interesting songs that had a history to track down. I first heard it the night before just by chance as performed by the Mood Kayo band, Hiroshi Wada & Mahina Stars（和田弘とマヒナスターズ）. It was quintessentially Latin Mood Kayo so I wanted to find out more about it, but when I looked up their file in J-Wiki, I couldn't see any hint of the title in their discography. (Unfortunately, the powers-that-be have taken down the Mahina Stars version above so it's been replaced by the instrumental version by guitarist Claude Ciari.)
So, it was off to YouTube to see if there were any more examples of the song since a few of the entries there sometimes would have some information via the uploader. I didn't find any other information on Mahina Stars' contribution but then I saw this performance by legendary Japanese chanson singer Fubuki Koshiji（越路吹雪）on the 1965 Kohaku Utagassen. As it turned out, she was the original singer of the Japanese version.
Yep, Japanese version. The very original of this song was "La Playa" (The Beach) by Dutch guitarist Jo Van Wetter which had been first released in February 1965. Supposedly the song garnered quite a bit of popularity through the world's radio stations, and I'm assuming that would include those in Japan since the late lyricist Tokiko Iwatani（岩谷時子）came up with the Japanese lyrics, and then it was up to Koshiji to interpret them.
Koshiji may have been more well known for her interpretations of French tunes but I think she also gave a great performance at that Kohaku (I was barely a month and a half old at the time) with that feeling of a slow contemplative tango. I'm currently unsure whether "Yogiri no Shinobi Ai" had ever been an official Koshiji single although it is known as one of her prominent songs, and I don't even know when the Mahina Stars cover came out unfortunately. Either of the Japanese versions though would be fine as musical accompaniment while sagely nursing that tumbler of whiskey.