火事で全てを失った高校生の月島龍聖。 　長らくひとりぼっちだったが、ある日、親戚のおじさんのすすめで義理の妹を迎えることになった。彼女もとある事故で両親を失ったという。 　名前は純菜（じゅんな）。 　歳は同じだが、義理の妹になった。 　ぼっち人生だった龍聖の生活が激しく、大きく変わっていく。 　二人で送る甘々の同棲生活。 　ただ、平和に過ごしたいと願う毎日。 　義理の妹がそばにいてくれれば、それでよかった――。 　High school student Ryusei Tsukishima, who lost everything in a fire, had been living alone for a long time. One day, at the suggestion of a relative, he decided to welcome a stepsister into his life. She, too, had lost her parents in an accident. Her name is Junna. Although they are the same age, she has become his stepsister. Ryusei’s solitary life begins to change dramatically and significantly. They embark on a sweet and cozy life together. Every day, he wishes for nothing more than peace. As long as his stepsister is by his side, that is enough for him.