After their midterm, the 10th graders of Yoo II go on a school retreat and arrive at a secluded retreat center located in the deep mountains. Then, a mysterious app is installed on the students' phones, and the deadly Mafia game begins. Not knowing what awaits them ahead, the students vote, not too seriously, for Heo Yul. However, when midnight strikes, Heo Yul kills himself, plunging everyone into a deep sleep along with the message "Night has come" on their phones. The next day, another friend is found dead, leaving students in confusion, but soon they realize the "Mafia Game" has become real.