Rumi Natsumi runs a phone sex service called Telephone Communication Madonna while attending high school, showcasing entrepreneurial initiative and organizational leadership. She personally manages difficult client cases, demonstrating confidence and a direct approach to unusual challenges.

When client Ryō Sugiura, who physically transforms into a destructive monster upon sexual arousal, contacts her service, Rumi becomes professionally and personally invested in his condition. Instead of retreating from his transformations, she develops a therapeutic regimen involving controlled exposure to erotic stimuli to help him manage his involuntary shapeshifting, reflecting her pragmatic problem-solving and growing attachment.

Her professional relationship with Ryō evolves into a romantic connection during their sessions, complicating her initial motives. This bond proves crucial when external threats arise, specifically the forcible kidnapping by delinquent classmate Oyuki Nohara. During the ensuing confrontation, Rumi physically defends herself against an attempted rape by Oyuki's subordinates using an improvised steel bar, highlighting her resilience under duress.

The climax occurs when Ryō's monstrous form becomes uncontrollable after he is sexually assaulted. Rumi intervenes by emotionally appealing to his humanity, successfully triggering his recollection of an alien entity's possession during a camping trip. This leads to Ryō expelling the entity and regaining control, showcasing Rumi's capacity for deep emotional connection and decisive crisis intervention.

Following these events, Rumi maintains her relationship with the cured Ryō and integrates Oyuki into her phone service operation, indicating pragmatic conflict resolution and the continuation of her entrepreneurial activities.