Misasa is a beautiful woman who works as a swimming instructor during the week. With an excellent figure and high professional reputation, she is well-liked by everyone around her. However, her life suddenly changes one day when her father, Detective Takuma, is murdered by a criminal he had arrested, and Misasa herself is violently assaulted. This traumatic event shatters her mind and pushes her to the brink of mental collapse. One day, standing before the mirror in her room, Misasa puts on a high-cut swimsuit and whispers, "Yes, this is it... this is what I need." A few days later, she appears again before the same mirror, now wearing a dominatrix-style mask, long gloves, thigh-high boots, and her beloved high-cut swimsuit. "This is the only thing left for me now..." From then on, she takes on the identity of "Swimsuit Mask," battling criminals at night while gaining experience as a heroine. Along the way, she resolves to avenge her father's death by targeting the criminal who killed him. Eventually, she discovers the location of his hideout. She infiltrates the base and confronts her archenemy, only to face an opponent far stronger than expected. After a fierce fight, she is defeated and captured. Her hands are bound, her swimsuit torn apart, and just before being assaulted again, the situation suddenly turns in her favor. Now it is the criminal who is restrained by Swimsuit Mask, forced to endure masochistic acts such as face sitting and spanking, and finally killed by having his neck squeezed between her thighs. [HAPPY END]