Kazuya is a 28 year-old divorced salaryman, who is convinced by his boss one night to go with him to a massage parlor. He does and meets a pretty young woman named Nagisa. He takes her card, but loses it when his aunt makes him help clean up the house in the front of the complex they live in (she's the landlord) so that a new boarder can come live with them.
Of course, it's Nagisa, and her 4 children, from toddler to teenager. Their father has died and she's doing everything she can to support these kids. Oh, and one more thing. She's only 21 years old, and may have never even consummated her marriage!

An 18-year-old university student Takanori and 17-year-old high schooler Hikaru become brother and sister when their parents get married. Just when their happy family life is about to start their parents' plane crash on their honeymoon! Despite this, Takanori and Hikaru continue to live together. However, everything changes as they learn about the billion yen insurance money their parents left for them! An adventure full of comedy and romance is about to begin!

A group of scattered people who were friends in high school start getting killed one by one, and danger soon follows the heroine, Aihara Kumi, as she starts suspecting that one of them could be the murderer.