Kuroe Akaishi is a lonely high school girl who has spent years avoiding her classmates due to a mysterious, incurable illness that causes frightening changes to her body. Because of this condition, she has been labeled Psycho-tan by her peers and has resigned herself to a life of isolation. Her quiet existence is upended when Arata Minami, the most popular and handsome boy in her class, unexpectedly begins to show a kind interest in her. As Kuroe finds her heart racing with new romantic feelings for Arata, she discovers that her illness has a monstrous secret: intense emotions, especially those born from love and embarrassment, trigger a dramatic transformation into a colossal kaiju.

The story is set in modern-day Japan, where the appearance of a giant monster is treated as a national crisis. Kuroe soon learns the truth from her devoted mother, Rinko, a former biologist who once discovered the remains of a real kaiju. Rinko confesses that Kuroe is not human but a kaiju herself, an egg she brought home and raised as a daughter. Struggling to accept this reality, Kuroe must navigate the perils of high school romance while hiding her dual identity from Arata, terrified that he will reject her if he learns she is the very monster causing havoc in the city. Her transformations are directly linked to her emotions; embarrassment causes monstrous appendages to sprout, and when her feelings reach a peak, she unleashes her full devastating form, often destroying parts of the city in the process.

As Kuroe attempts to balance a normal relationship with her explosive secret, several key characters enter her life. She forms an unlikely friendship with Manatsu Tomosato, a wealthy, eccentric girl who has a fanatical romantic obsession with the kaiju and dreams of being stomped flat by it, unaware that her new best friend is the object of her desire. Kuroe also grows closer to the popular influencer Raimu Kono, who helps her gain confidence, while her overprotective mother and a concerned former colleague work to protect her from government scrutiny. A major narrative arc involves Kuroe's struggle with self-acceptance as her relationship with Arata deepens. He is drawn to her not in spite of her strangeness but because of her genuine nature, though he remains oblivious to the truth, leading to comedic and dangerous misunderstandings where he finds himself trying to communicate with the very monster that is his girlfriend.