One evening, in a moment of alcohol-fueled candor, Satsuki drops a bombshell: "To be honest... back in school, I actually had a crush on Kodai."
Delighted, Kodai admits in turn, "Truth be told, I always had my eye on you, too."
Ria follows right after, confessing, "Actually, I always had a thing for Yuto, as well."
Caught up in the sudden commotion, Yuto also confesses that he had secretly harbored feelings for Ria.

"Well then, why don't we try kissing each other?" Ria playfully proposes in the heat of the moment.
Satsuki readily agrees, saying, "Sounds good to me," and leans in to kiss Kodai.
Unable to look away or stop them, Yuto finds himself watching his own wife kiss his friend.
Right after, Ria pulls Yuto into a kiss as well. Catching glimpses of each other kissing right before their eyes, a heady rush of excitement surges among the four.

Carried by that momentum, things soon escalate into partner swapping...

Afterward, the topic comes up of whether they should continue the arrangement going forward. Having awakened to the taboo thrill of swapping, every single one of them agrees.

Yuto, Satsuki, Kodai, and Ria.
Thus begins the four friends' partner-swapping life together.