Big sister Chika is worried about her younger brother, who lacks sexual knowledge and is inexperienced. During a family dinner, the atmosphere turns awkward when the brother brings up seeing their parents having sex the previous night. Wanting to diffuse the tension, Chika feels a sense of duty to educate him properly and decides to use her own body to carefully guide him through sex education. She patiently explains everything—from correctly putting on a condom to the mechanics of intercourse—helping her brother deepen his understanding step by step.