On a cold, winter day, Sang-Min (Jeon Do-Yeon) asks Ki-Hong (Gong Yoo) for a light. The two strangers have dropped their kids off at a pickup area for a children's camp in Helsinki, Finland. A spark of mutual interest is ignited between the man and woman. The man and woman decide to trail their children to the campground. On their way back, a snowstorm forces them to spend an evening at an inn. In the morning, they walk through the woods and to a secluded sauna. They leave the next day without knowing each other's name. Several months later, Sang-Min fixes a display at her workplace and sees Ki-Hong walking by. The man and woman are both married, but share a bond that defies their condition.