Just by existing, Kita-kun, a "genius kept man," effortlessly captures the hearts of people of all ages and genders. A mysterious man with an unknown background and past, he accepts everyone who comes his way. Three individuals—Minami, Toko, and Nishino—fall in love with him at the same time.
Rather than increasing the number of rivals, the three come up with a bold idea: “If we can’t give him up, why not share him?” They form an alliance called the “33% Club,” each claiming to be 33% of Kita-kun’s lover. Thus begins a strange four-person cohabitation.
They establish rules so that each person gets exclusive time with Kita-kun on specific days of the week, with no sneaking around allowed. Amid the chaotic lifestyle, the pasts of the three lovers gradually come to light. Whether intentional or not, Kita-kun helps them confront and resolve their personal struggles with his casual words and natural charm.
Just as the four begin to see their shared life as irreplaceable, mysterious letters mocking their unusual relationship start circulating around their apartment and workplaces, threatening the continuation of their cohabitation.
To make matters more complicated, each of the three begins to develop feelings for someone else. Despite their deep love for Kita-kun, they start to question their relationship with him—especially since he never tries to hold any of them back. As Kita-kun’s own past is revealed, the dynamics among the four begin to shift.