Recently, the older sister has gained weight and can't wear her pants. She asks her younger brother for help, but the younger brother's mischievous Kokoro is ignited and he puts his hand inside her panties and moves his fingers. From here, the younger brother gradually escalates and tricks his serious and innocent older sister into doing super perverted play such as "Let's do deep throat to make her face smaller," "Let's shake our hips in cowgirl position to make her waist smaller," "Let's do missionary position to train our abs," and "Let's drink semen to make our skin smooth."