A young woman named Soo-hyeon, Chae Min-seo, is battling cancer, and her mute older sister Ji-hyeon, Yu Seon, decides that Soo-hyeon should come home to enjoy what little time she has left rather than spend it in a hospital. To help her cope, Ji-hyeon buys a long-haired wig so Soo-hyeon can go out without feeling ashamed of being bald, and Soo-hyeon embraces the gift with contagious happiness, wearing it constantly and beginning to look and act healthier, more confident, and more radiant as she ventures out into the world.
Soon, however, Soo-hyeon grows vain and becomes fixated on the wig, even attempting to win Ji-hyeon’s boyfriend, Kyeong-joo, Sa Hyeon-jin. He ends things with Ji-hyeon, showing little interest in Soo-hyeon. When one of Ji-hyeon's friends discovers her husband’s infidelity, Soo-hyeon lends her the wig, insisting it will boost her confidence and beauty. The next day, that friend is found dead with her husband, both covered in hair, and the wig is mysteriously returned to Soo-hyeon. Frightening visions of the friend's demise haunt Soo-hyeon, and she cruelly ignores Ji-hyeon’s attempts at conversation, while disturbing images plague her mind. She even tries to sleep with Ji-hyeon's ex, but Ji-hyeon intervenes, and the two drive home as Soo-hyeon goads her.
Back at home the sisters fall into silence until Soo-hyeon washes the wig and Ji-hyeon spots a ghost in it, revealing that the wig is cursed and corrupts Soo-hyeon. Ji-hyeon takes control, seizing the wig, locking Soo-hyeon in her room, and cutting the cursed item to shreds. The cancer returns with renewed force, and Soo-hyeon is forced back into the hospital, where she escapes by skipping her medication, leaving a disturbing image of her pulling pills from a bloody scalp.
The truth behind the haunting emerges: the wig belonged to a man who had been in a romantic relationship with Ji-hyeon's ex Kyeong-bin Rah. As a gay man, he faced shame from Ji-hyeon's ex, was beaten by a group of teens, and after his hair was cut, he took his own life. The ghost of this man begins to corrupt and possess Soo-hyeon, guiding her to seek out Ji-hyeon’s ex, and the two reconnect. As they share a kiss, Soo-hyeon's hair grows at an impossible speed, and Ji-hyeon arrives to set fire to the strands, freeing her sister and destroying the ghost. Yet a terrifying twist follows: Ji-hyeon hallucinates that the ghost is still with them and, believing she is protecting Soo-hyeon, strikes her sister. Only then does she realize it was a hallucination, and she collapses in tears at what she has done. The film ends with a photograph of the two sisters as children, a reminder of their bond before cancer and Ji-hyeon’s loss of speech.
