Hyo-jeong is a co-host of a shopping channel show lingerie. However, she loses her job to her younger, more glamorous, rival, and she also gets into a conflict with her boyfriend, Dong-hoon. Deciding to calm herself, Hyo-jeong heeds to an old friend, Seon-hwa's advice to take up yoga class for a week. Her yoga colleagues include the jolly but demented In-soon, the failing actress Yeon-joo, the troubled mother Yoo-kyeong, and the quiet Bo-ra. The class is taught by the cryptic instructor Na-ni, who states the following rules: 1. Not to eat, 2. Not to shower within an hour after training, 3. Not to communicate with the outside, 4. Not to look at mirrors, and 5. Not to go outside. Na-ni also informs the class that the owner of the yoga school, the former 1970s actress Kan Mi-hee, will join the class on the seventh day.
The five women are suspicious that something is amiss with the school and Na-ni herself. Nevertheless, they continue to undergo intensive training and even start to bicker with each other as they attempt to achieve Kundalini (the coiled "serpentine" energy) as promised by Na-ni. Meanwhile, Dong-hoon, who likes to watch Mi-hee's films during her heyday, tries to contact Mi-hee's frequent collaborating director, Kang Hee-joong, only to find him dead with his eye gouged out in his house. Recognizing Seon-hwa, whom he met just before he discovered Hee-jong's corpse, as Hyo-jeong's friend, he confronts her with Hyo-jeong's whereabouts. Refusing to answer, Seon-hwa heads down the street only to be hit by an incoming car. Dong-hoon finally contacts a janitor who formerly worked in Hee-jong's studio, learning that Mi-hee might have something to do with the death of her substitute in her final film, after she was fired.
Throughout the week the women start to break the rules: on the third day, Yoo-kyeong bathes less than an hour after training and In-soon eats a strangely prepared dinner. Yoo-kyeong is soon found lifeless with slash marks and black goo in the bathroom. Na-ni takes her in to treat her, but the latter is choked by Na-ni's pet snake. Later, Bo-ra lends Yeon-joo her mirror, and both of them are dragged by an unseen force on the fifth day. On the sixth day, In-soon catches Hyo-jeong calling Dong-hoon and tries to report it to Na-ni, but she receives her punishment for lawbreaking on the third day: she is revealed to have eaten her pet mouse's corpse which damages her skin, and she is dragged away by a black-clothed woman.
Becoming the only surviving pupil on the seventh day, Hyo-jeong meditates and gets glimpses of Mi-hee. She bolts away to find Mi-hee's corpse with snake-like skin and eye producing a snake. Hyo-jeong is knocked out by Na-ni, who then performs a ritual for Mi-hee to take Hyo-jeong's soul. Hyo-jeong, however, manages to escape and discovers her colleagues' dying bodies. As she works to get away from Na-ni, flashbacks reveal that Na-ni is an admirer of Mi-hee and wants to be exactly like her. She says that Mi-hee is a demon who trades Kundalini in exchange for souls. Stating that she cannot continue this anymore, Na-ni succumbs to her wounds inflicted by Mi-hee while Hyo-jeong escapes.
Hyo-jeong is able to reconcile with Dong-hoon. As she heads back home, she encounters her colleagues at the subway, all mysteriously alive. After a brief power failure, Hyo-jeong screams when all people disappear, leaving only posters depicting Mi-hee hanging on the walls.