姐さんカウントダウン!

School girl Riko Umekita's life is thrown upside down by scariest boy in her class, Saruwada-kun. His family is from a long line of Yakuza and is known not to mess with them. She is dragged home with him only to find her parents there after some circumstances... Saruwada-kun then asks her to marry him. What is going on?