Kayo Asakura lives an ordinary life as a student at Tokyo's Meisei Academy. This routine shatters when an extraterrestrial object crashes onto campus, leaving a crater. Unaware of the danger, she continues spending time with her boyfriend, Tsutomu Kawai, and friends. After Tsutomu falls into the crater, he becomes possessed by an alien entity, a change that remains hidden at first. Kayo witnesses horrifying attacks by monstrous entities, first at her friend Yoshiko's home and later at school where her English teacher, Miss Yamazaki, is assaulted. She fights back, temporarily repelling one attacker with a thrown fire extinguisher. Still unaware Tsutomu is the source, she interacts with him until he transforms into his true alien form and assaults her, resulting in impregnation by the entity. An interplanetary agent, Professor Kasu, erases her memories of affection for Tsutomu, though the pregnancy persists despite her return to apparent normality.

The pregnancy advances unnaturally fast. Kayo gives birth at home with her friend Miki present. The newborn demonic infant immediately attacks Miki and matures into a full-grown demon beast within hours. Kayo later encounters this offspring in a hospital maternity wing, where it creates mutated infants. Her connection to the beast intensifies when the agent, now inhabiting a bike gang leader's body, confronts her, forcing the conjoined beast to partially reveal itself. Separation attempts risk her life. The demon beast takes Kayo back to the origin crater, revealing its kind originated on Earth and seeks to reclaim it. Following a confrontation between the beast and the agent, both vanish, teleported to another dimension, leaving Kayo behind.

Some time later, Kayo attends a pop performance by classmate Hitomi. She suddenly experiences nausea and sprouts a tentacle-like appendage, using it to assault Hitomi against her will. This possession escalates when Hitomi is forced to expose herself on live television. The exiled demon beast interrupts the broadcast through Kayo's body, declaring war on humanity and signaling her complete loss of autonomy. Kayo's physiological compatibility makes her a critical target for the demon beasts' plan to create human-alien hybrids capable of surviving on Earth. This biological significance drives her traumatic experiences, including repeated violations and the rapid birth of hybrid offspring. Her transformation from ordinary student to vessel for the demonic invasion underscores her role in the conflict between the aliens and those trying to stop them.