Cherie, a young single mother and pre-law student, works as a secretary at a legal firm in Los Angeles. Her day takes a turn when her boss, James R. Fuller, calls to inform her that she has double-booked him for an important client dinner at the same time as his anniversary dinner. With no recollection of the conflicting schedule, Cherie reluctantly agrees to attend the dinner on his behalf.
As she gets ready, Cherie's period unexpectedly starts, leading to the realization that she is out of tampons. Once she meets her boss's client, Ethan, at his home, she finds herself captivated by him, though things take a dark turn when he reveals a history of trauma related to dogs during a sudden outburst. Despite the tension, Ethan offers Cherie to spend the night, setting an alarm for her early departure before her daughter wakes up. However, she notices an earlier alarm set for 5:25 AM, raising suspicions as she decides to stay for a drink.
Tragedy strikes as Ethan suddenly attacks Cherie, forcing her to flee down the street in desperation for assistance. When she finally finds a couple of women outside a movie theater, they agree to call 911. Unfortunately, upon the police's arrival, Cherie is arrested for public intoxication, even while protesting that she had been assaulted. In her cell, she speaks with another inmate who panics upon hearing Ethan’s name, warning Cherie about his manipulative nature and urging her to seek help from the First Lady.
After being released, Cherie goes to James’s apartment, where he appears to be sympathetic and offers her clean clothes. In her quest for information about Ethan, she discovers shocking details on James's computer: she’s merely the latest victim in a long series targeted by Ethan, who can sense her blood according to Judy, James’ wife. In a panic, Cherie flees on a bus and calls her ex-boyfriend Trey for assistance.
Stopping at a gas station to buy tampons, Cherie is ambushed by Ethan. She manages to fend him off momentarily and escapes with Trey, who drives her to his place, complicating the situation as his new girlfriend, Dawn, Cherie's former best friend, is present. Remarkably sympathetic, Dawn tries to help when Ethan arrives looking for Cherie, but the confrontation ends tragically with Ethan easily overpowering Dawn and her friends.
In a desperate bid for survival, Cherie recalls Judy’s advice and uses her menstrual blood to mislead Ethan. She finds refuge in a church, requesting holy water and a crucifix, only to discover the priest’s lifeless body, revealing Ethan’s ability to impersonate him. As the real Ethan confronts her, an opportunity arises when he is attacked by the priest in his true form, allowing Cherie to escape.
Cherie stumbles into an underground rave where she comes across a flyer for the First Lady and promptly contacts them, learning of a nearby spa. During the chaotic environment, she is rescued by a group of party girls who befriend her. However, as Ethan's alarm rings for sunrise, he finds them, but Cherie is saved by a barking pitbull that forces Ethan to vanish.
At the spa, she meets Dinah, the First Lady, and a group of women who practice martial arts. Dinah discloses Ethan's origins as a fallen angel, revealing his sinister plan for male domination throughout history. To take him down, they need Cherie to act as bait. After a thorough cleansing, Cherie heads to the Santa Monica Pier, where she intentionally reopens her wounds to draw Ethan's attention.
As he attempts to finish her off, Ethan is caught off guard by a rock thrown through a window, exposing him to sunlight, which weakens him severely. The other women from the spa join the fray, bombarding the windows until Ethan is fully incapacitated by the blaring sunlight. Cherie seizes the moment to mock Ethan in his weakened state, and Dinah sets him ablaze, ensuring his end. Victorious, Cherie returns home to safety, finally able to embrace her daughter once more.
