18-year-old little sister Natsu is a girl with a slightly naughty side. When she's short on cash, she tends to mischievously ask guys for help in exchange for intimate experiences. Today, Natsu was turned down when she asked her older brother for pocket money. Desperate for cash, she decides to tempt him with a little seduction. With an irresistibly cute invitation, she whispers, "If you give me allowance, you can do whatever you want with me." Faced with such a tempting offer from his adorable little sister, how could any brother possibly refuse?