uchi no otouto maji de dekain dakedo mi ni konai

A boy named Nao, a young preteen, is caught masturbating in the bathroom by his older teen sister Chiaki after having broken his beloved sex toy, Sakura-chan. Far from disturbed, Chiaki is surprised by the size of her younger brother's penis and takes a photo of it to share with her friends, urging them to come over to see his "Cockzilla."

The very next day, two girls named Nagisa and Yukiko arrive at Chiaki's house to look at Nao's Cockzilla. Shamelessly, Chiaki exposes her brother's body before her friends' astonished eyes while they start discussing his size. With the intention of knowing its true caliber, they engage in stimulating it beyond its limits.