Yuuta's sister Yuri finally entrusts her daughters to him as she and her husband Shingo leaves for a business trip-cum-honeymoon, with an instruction never to bring his girlfriend into the house. Coming home after seeing the couple off at the airport, Sora and Miu then introduce him to the house rules. Everything goes smoothly for the rest of the day well into bedtime until news came that the plane Yuri and Shingo were in crashed and they were among those considered missing. The family decides to break the siblings up, but Yuuta, remembering how his sister raised him herself after being orphaned, decides to take them all with him in his apartment.
帰宅した姉から、夫の出張に同行するため、さらに一週間の留守番を頼まれる祐太。
無茶だと一度は断るが、遅い新婚旅行だから行かせてあげたいという空と美羽の声に押されて請け負うことに。
だが、姉夫婦の乗った飛行機が事故に遭い、状況は一変。
夫婦が行方不明のまま葬儀が行われ、空、美羽、ひなはそれぞれ信吾の親族に引き取られることになった。
離れたくないと必死に訴える彼女たちに昔の姉の姿を重ねた祐太は、自分が引き取ると宣言する。
누나가 유타를 부른 진짜 이유는 집을 비우는 일주일간 세 자매를 봐달라는 것이었다. 고민하던 유타는 이를 수락하고, 소라 미우 히나 세 자매와 요란하지만 즐거운 하루를 보낸다. 하지만 모두 잠든 사이 비보가 날아드는데...
Yuuta's sister Yuri finally entrusts her daughters to him as she and her husband Shingo leaves for a business trip-cum-honeymoon, with an instruction never to bring his girlfriend into the house. Coming home after seeing the couple off at the airport, Sora and Miu then introduce him to the house rules. Everything goes smoothly for the rest of the day well into bedtime until news came that the plane Yuri and Shingo were in crashed and they were among those considered missing. The family decides to break the siblings up, but Yuuta, remembering how his sister raised him herself after being orphaned, decides to take them all with him in his apartment.
