It's been five years since I married my wife. Occasionally, my wife's younger sister, Ruka, comes over to our house to visit, adding unexpected moments to our daily life. One day, I drank too much at a company drinking party and returned home intoxicated. Riding the wave of alcohol, I spent some intimate time with my wife—the first in a while. But the next morning, as I woke up with a hangover, I saw my wife and Ruka happily chatting together while preparing breakfast. A sharp pain pierced my chest. Was it my wife I was with last night—or was it Ruka? Strangely, the memories of that night are already fading away.