Hiroki lives with his pregnant wife, Mari, in a cramped apartment. Upon entering her final month of pregnancy, Mari's older sister, Rio, decides to stay over for a while to help with household chores. From the moment they meet, Hiroki is smitten by Rio's charm, his nose practically dragging on the ground. Before long, Mari suddenly goes into labor and is rushed to the hospital! The baby is born safely, and back at home, Hiroki and Rio raise a toast in celebration. Drunk and devoid of reason, the two find themselves alone in the house without Mari…
