古くなった実家を２世帯住宅に建て替えることにした立花家だが、ひきこもりの兄・雅治（松田龍平）の居住スペースが問題に。父・修（木場勝己）はこれを機に自立すればいいと言い、母・恵子（風吹ジュン）は無理をさせたくないと擁護する。一方、妹・沙織（臼田あさ美）は兄の部屋をなくして玄関を二つにしたいと自分勝手な主張を繰り出す。話し合いがまとまらない中、ハウスメーカーの川村（井之脇海）の説得によりなんとか「２．５世帯住宅」の建設に向けて歩み出す。
そうして始まった新生活。ゲームの世界ではカリスマとあがめられている雅治だが、妹には「家事を手伝え」、中学生になるお年頃のめい・恵麻（白鳥玉季）には「キモい」と言われ、仕事をしていない“０．５”の存在に居場所はありそうになく……。
The story revolves around the Tachibana family, who live in a 2.5-generation household. In this type of household, the parents and their children live together in a two-generation home, with the addition of a single sibling from the younger generation. The Tachibana family decides to rebuild their old home into a 2.5-generation household, and the 40-year-old unemployed and reclusive protagonist, Masaharu Tachibana, ends up living with his sister and her family.
Although Masaharu is revered as a gaming expert in the virtual world, he faces challenges in his new living situation as his sister asks him to help with household chores, and his teenage niece rejects him as "weird." Having lived life at his own pace until now, Masaharu gradually learned to interact with people and regain a more social lifestyle amidst the changing environment.
