A group of tired commuters have caught the last train home, ready to hop into their beds for the night. However, their train soon veers off-course and crashes into an abandoned tunnel. The three surviving commuters; withdrawn mother Yi Ling, her son Lucas, and student Janice, are forced to band together and survive the night, as well as avoid the mysterious monster hunting them. Yi Ling still suffers from flashbacks to the car accident that killed her husband and nearly took Lucas life too. When Lucas is snatched away the monster, her maternal instincts are tested, and Janices ideas of family are challenged as she remembers her difficult relationship with her estranged father. Back at the subway control headquarters, the engineers do their best to make sense of the situation and rescue the survivors.