Having an unwavering belief in his elite status, Kou Ichinomiya leads a privileged life as the son of a wealthy businessman. He is determined to evade being indebted to anyone, but fate has other plans. A single encounter with mischievous kids on Arakawa Bridge results in him tumbling into the water beneath. Fortunately, a passerby rescues him from drowning, and Kou finds himself indebted to this stranger. Seething with anger, Kou insists on repaying her, but it might just be the worst deal he's ever made. The stranger, known as Nino, is a homeless girl who resides in a cardboard box beneath the bridge, donning a stoic expression and tracksuit. All she desires is to experience love. She requests Kou to be her boyfriend, giving him no opportunity to refuse. Inevitably, Kou is forced to abandon his cushy mansion and embark on a new life under the bridge alongside Nino.