Dr. Richard Jacks, a talented perfumer working at a prominent fragrance company, faces significant professional frustrations. Despite his efforts, his recent perfume projects have all failed, and his chief executive, Mrs. Unterveldt, dismisses his latest creation, claiming that it sounds more like a woman's perfume and insisting that a woman should be working on it. After the passing of his great-uncle, Richard attends the reading of the will and discovers that he inherits nothing but old notes from scientific experiments, along with the revelation that he is the great-grandson of the infamous Dr. Henry Jekyll. Intrigued and motivated by this discovery, Richard decides to experiment further with his ancestor's formula.
He begins by adding more estrogen to the original potion in hopes of creating a better, more effective serum. After ingesting the serum, Richard waits anxiously but notices no immediate change. However, later that day, a series of small incidents trigger his transformation. While attending a job interview at a restaurant, a waiter spills coffee on him, and in his excitement over a potential new job and higher salary, the serum's effects are suddenly unleashed. Richard's body begins to change: his fingernails grow long, his arm hair disappears, his voice starts to shift, and notably, his male reproductive organs transform into a vagina inside his pants. To his shock and horror, his hair begins to grow out rapidly, and when the waiter notices his changing chest and attempts to help, he dabs Richard's chest, causing swelling in both sides of his chest. Panicking, Richard escapes from the restaurant and rushes back to his laboratory.
As he struggles to find a mirror, the last thing he sees before ripping off his shirt is a perfect pair of breasts forming on his body, confirming the full extent of his transformation. This unexpected change leaves Richard both bewildered and frightened, as he fully realizes he's become a woman. Later, in the laboratory shower, Pete discovers her in her transformed state. She caresses herself seductively, looking confidently at Pete before stepping out of the shower, adopting the persona of "Helen Hyde." Under this new identity, she convinces Richard's colleagues that she is his new assistant. Helen starts to subtly manipulate her surroundings: she contributes to reports, flirts with her superiors, Yves Dubois and Oliver Mintz, and even rewards herself with a shopping spree paid for with Richard’s credit cards.
Helen then befriends Richard's fiancée, Sarah, and persuades her to move out of Richard's apartment, further solidifying her influence over Richard’s personal life. When Richard returns to work, he notices that his bosses are already impressed with Helen, who has seamlessly integrated herself into his professional environment. Realizing he doesn’t remember transforming into Helen, Richard becomes increasingly confused and troubled, especially when he discovers that Sarah has moved to his cousin Larry's place.
Determined to reconnect with her, Richard visits Sarah, who, despite being upset with him, admits she finds both his cousin and Helen "fantastic and so alive." Richard manages to get her to agree to dinner, but during their meeting, he begins to transform again, causing Sarah to leave in confusion. Helen, now in a seductive leather dress, approaches her work and inquires about a promotion with Oliver Mintz, who dismisses her as merely a secretary and denies any current vacancies.
Pete, seeing Helen leaving Mintz's office, tries to flirt with her by offering her a chance to help with a new scent. Initially dismissive, Helen soon agrees after mixing sulfuric acid into his perfume bottle, offering Pete a warning: > "Splash a little bit of this on." She then takes a puff of her cigarette, smirking as she hears Pete's screams in the distance, showing her ruthless side.
Helen then sets her sights on Oliver, seducing him and convincing him to indulge her demands, which leads to a night at his apartment. However, the plan begins to unravel when her breast deflates unexpectedly during an intimate moment. Frightened, she locks herself in the bathroom, but Oliver mistakes her distressed sounds for an orgasm and tries to force his way in. Richard escapes during this chaos, and Oliver later informs everyone that Helen will be officially taking Richard’s role.
In a desperate attempt to stop Helen, Richard attempts to handcuff himself to prevent her from leaving, but before he can succeed, his fiancée, Sarah, finds him in this compromising state and mistakenly believes he's a cross-dressing pervert. Minutes later, Richard finally manages to free himself but immediately reverts back to Helen.
Back at the office, Helen, now dressed professionally, prepares to launch her plan to dominate the fragrance industry. She steals Richard’s perfume formula, renames it Indulge, and uses her feminine charm to influence the company’s leaders, Yves Dubois and Oliver Mintz. During a crucial meeting, Helen’s suggestive behavior and persuasive tactics lead both men to become infatuated with her. She uses her allure to push the fragrance’s approval through, subtly manipulating her male colleagues under the table and ensuring her rise in the company.
Meanwhile, Richard, in an attempt to regain control, creates videotapes revealing her true intentions: she plans to fully take over his body. He tries to stop her by stripping naked in his office and writing obscene messages on his body, but he misjudges the timing of his transformation and ends up getting fired. Helen also intercepts a call from Pete, who suspects her theft of his scent formula, and she electrocutes him under the guise of a stranded driver.
Desperate to end Helen’s reign, Richard, with the help of Sarah, devises a new serum intended to reverse the transformation. To prevent her escape, Richard immobilizes himself on a bed, but Helen manages to escape and attend her perfume launch party. Sarah follows her and discovers that the serum is gradually returning her to Richard’s original form. At the celebration, Helen attempts to flaunt her success, but Sarah intervenes and injects her with the remaining serum. As Helen undergoes one final transformation before the audience’s eyes, Richard reappears fully restored.
In a heartfelt speech, Richard admits that he was truly Helen all along, explaining that his transformation allowed him to understand women better—"I needed to become a woman to understand them." His boss, recognizing his dedication and the strange journey, rehires him with a promotion and a vacation to recover. As the scene closes, Richard walks out of the party hand-in-hand with Sarah, finally at peace with his true self.
