Yui and her family live above the barbecue restaurant her father owns. 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 bully and, as he becomes a teenager, develops extremely severe acne from the grease in the air and his habit of drinking oil straight from the can. He is bullied at school because of his skin condition and extremely violent temper. One day, Goro orders Yui to go out and buy more oil for him to drink when they run out. She hesitates and he traps her, bursting all his zits at once so that pus rains down onto her. He goes into a psychotic rage and screams that he's going to kill everyone. However, their father saves Yui by beating Goro to death with a frying pan. He serves Goro's flesh to the customers in his restaurant.
Yui begins to have a recurring nightmare in which Mt Fuji (which is visible from outside her home) erupts grease, covering everything in grease. 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. Eventually, Yui catches her father cutting off his own leg to serve in the restaurant since Goro's been eaten and she refused to drink the oil. As the saturation level hits 100% she observes that her father's leg is not leaking blood; only grease.