The core story of Shinseki no Ko to O Tomari revolves around a familiar and often controversial trope within certain niches of adult entertainment: a situation involving a relative's child staying over. The specific narrative often centers on a woman in her thirties who is asked to take care of her relative's... (with the remainder of the sentence typically filled by various interpretations of the familial or suggestive dynamic).
The narrative follows a classic adult romance/H-anime setup: a male protagonist ends up sharing a living space or bedroom during a family visit or overnight stay. It leans heavily on the "cohabitation" trope popular in niche romance manga and short-form adult OVAs. Navigating the Show on MyAnimeList (MAL) shinseki no ko to o tomari da kara mal