A sudden blizzard traps Su Nian'an in apartment 503. A blank-avatar doomsday warning in the owners' chat shatters the peaceful life. Strange noises come from next door, midnight knocking follows one after another, and mysterious direct messages continuously issue warnings to her. To withstand the extreme cold crisis, she guards step by step against the malicious people around her, struggling to survive in the snow-shrouded building. Repeated experiences make her feel doubtful, and when she pries open the door and flips through the diary, she is shocked to realize that the one who has been guiding her in secret is actually another herself trapped in the cycle. Trapped in reincarnation, she will ultimately face her inner choices.