Doing It With My Girlfriend's Younger Sister Until Morning As We Both Sheltered From The Rain - Rin Kira

The movie revolves around the forbidden relationship between the main male character and his girlfriend's younger sister, who takes shelter at his place during a rainstorm. As they spend more time together, they develop a strong physical attraction, leading to a secret affair. The central dramatic tension lies in the illicit nature of their relationship and the potential consequences of being discovered, particularly by the girlfriend.