Yui and her family live above the barbecue restaurant her father owns, in a town at the foot of Mount Fuji. The shop and restaurant are covered in oil and grease, and the air is constantly heavy with grease due to a lack of ventilation. Yui becomes sensitive to what she calls the "saturation level" of grease in the air, which currently stands at 50%.
Yui's older brother, Goro, is a violent and abusive bully. As he becomes a teenager, he develops extremely severe acne from the grease in the air, puberty, and his habit of drinking oil straight from the can. He is bullied at school because of his skin condition, which makes his already extremely violent temper worse. One day, Goro orders Yui to go out and buy more oil for him to drink when they run out. Although she hesitates, he gets angry and he traps her, bursting loads of his zits on his face at once so that pus rains down onto her. He goes into an extreme psychosensitive rage and he screams he's going to kill everyone in revenge. However, their father saves Yui by bashing Goro over the head with a frying pan, accidentally killing him; he then gets rid of the body by serving Goro's flesh to the customers in his restaurant.
Yui loses her ability to take pleasure in the sight of Mount Fuji after she starts to have a recurring nightmare in which it erupts grease, flooding their town. The "saturation level" also rises to 80%. The restaurant enjoys a revival in popularity due to Goro being served to the patrons but, when the meat runs out, the customers stop coming back. Yui begins to develop acne and a bad temper, just like Goro. She wakes one night to find her father forcing oil down her throat just as Goro used to drink it. From then on, Yui can't sleep due to her father's constant efforts to break into her room and give her oil.
With no more meat left, Yui's father is forced to close the restaurant. He begins drinking the oil himself. His skin and hair become even greasier, and the saturation level continues to rise. The entire house is now full to the brim with grease, and grease drips constantly from the ceiling the saturation level is 90%. Eventually, Yui catches a glimpse of her father cutting off his own leg to serve in the restaurant since Goro's been eaten and she refuses to drink the oil; she observes that her father's leg is not leaking blood, only grease, as she realizes the saturation level is now at 100%.