Masato Ooizumi has just entered high school and finds himself staying at the home of a relative. Living there is his older female cousin, a "big sis" figure who quickly begins getting far too close for comfort, blurring the lines between familial affection and romantic interest. The story follows the slightly risqué romantic comedy that unfolds as they start their new life together under one roof.

The main character is Masato Ooizumi, a teenage boy navigating the start of his high school career and the confusing dynamic with his cousin. His counterpart is his unnamed older cousin, whose overly affectionate behavior drives the plot. The setting is primarily the shared home where the two live together, with the central conflict stemming from the intimate and "slightly naughty" nature of their new cohabitation arrangement.

The manga is written and illustrated by Kyouhei Igarashi. The overarching narrative arc focuses on the development of the unusual relationship between the cousins as they adjust to living together, with the story structured around a series of everyday situations that become charged with romantic and comedic tension. Information about specific, named narrative story arcs beyond the core premise is not available.