SAI means "again" in Japanese, like the prefix "re-". In this 3rd volume of Heaven and Demon, Aoki and Tengoku start dating and slowly deepen their relationship. But one day, when Aoki is talking with his friend on speakerphone while cooking, his friend brings up what Aoki said some time ago about having no plans to marry his current partner. Tengoku hears but says nothing about it. When Tengoku has to leave after dinner, Aoki walks with him to the station with a slight feeling of uneasiness. They kiss on the way, on the dark, lonely street at night. Aoki then notices someone passing by that could have seen them kissing, so he unintentionally says "...This sucks". Now Tengoku suddenly changes his tune and walks away, leaving Aoki alone and perplexed. Why did Tengoku leave like that? Their good relationship now seems to be in crisis...

Official English Translation:
MANGA.CLUB | Manga Planet (futekiya) | MangaPlaza | Renta! | Coolmic (webcomic)