Newlywed Hiroyuki lives with his wife Yumi and her parents, but one day he falls out of favor with Yumi due to an incident involving his coworker. Trying to reconcile, he calls out to her from outside the shower door—only to find his mother-in-law, Mari, standing there when the door opens. Flustered, he quickly apologizes, but not before catching a clear view of Mari's lush, seductive big tits. Surprised by the unexpected, alluring figure hidden beneath the gentle demeanor of his mother-in-law, Hiroyuki finds himself drawn into a forbidden desire. What will become of the taboo feelings he can no longer suppress?