Kaito gradually began to feel anxious one day, all because of his sister-in-law Rena's strange behavior. He was grateful she had come to help with housework while his wife was away on maternity leave. But her actions afterward were incredibly odd. She would enter the bathroom naked, claiming she just wanted to wash his back... It was as if she were hiding something. For a moment, Kaito wondered if she was actually coming on to him, but quickly snapped back to reality, assuming it was just a joke. Still, the uneasiness lingering deep in his chest refused to fade. As night fell and he slipped under the covers, he found himself caught in an unexpected situation.