Se-jin (Ko So-young) lives alone in a room in a high-rise luxury apartment. Late one night, while waiting for a train at a subway station, a mysterious woman suddenly grabbed her and tried to jump onto the tracks together. Before she could recover from the shock, the woman had already died. In addition, she had a conflict with her partner at work over how to decorate the window display for the upcoming Christmas, and she began to be tormented by nightmares every night. The only thing that could be considered happy was that she met a disabled girl named Yoo-yeon (Jang Hee-jin) who lived in the apartment opposite while jogging in the morning one day. Yoo-yeon said that although both her parents had passed away, she was living happily thanks to the care of a kind neighbor, and at the end of the conversation, she gave her a Rubik's Cube. Se-jin noticed that there was something strange in the room where Yoo-yeon lived, and then used a telescope to observe every night. She also found that every time the time reached 9:56, the lights in the entire apartment would be turned off, and a body would be dragged out the next morning. Se-jin's behavior was discovered by a female high school student (Park Ha-sun) and she told her mother, who called the police, but Se-jin did not take the police's warning seriously. Soon, the high school girl took the initiative to apologize to Shizhen, and the two became friends. However, her repeated failure to solve the Rubik's Cube made Shizhen discover the secret related to Rouyan hidden in it, and she gasped.