The Jo brothers, Jo Gyu-bin (uke) and Jo Eun-bin (seme), grew up in a family envied by others, yet their relationship was so strained they hadn't had a proper conversation in over 10 years. Then, a sudden accident leaves Eun-bin with amnesia, regressing to the mental age of an elementary schooler. Gyu-bin takes responsibility for the accident and begins to care for Eun-bin.

Struggling to cope with his adult-bodied brother who acts like a young child—throwing tantrums, sitting down in the middle of walking, and clinging while crying—Gyu-bin is startled when Eun-bin secretly visits his room late one night.

"Hyung, my pee-pee hurts."

Eun-bin whimpers, unaware of what an erection is or how to relieve the urge through masturbation. Gyu-bin, flustered, looks at him, and partly out of anger, ends up having sexual relations with him.

Gyu-bin realizes his mistake but initially takes it lightly, thinking, "I'll just play with him until his memory comes back. Even if it does, he'll be too embarrassed to tell anyone anyway." However, this mindset quickly pulls him into a mire he can no longer escape.