One day, Lee Won-yeong, an ordinary college student, loses his parents in a car accident. He almost dies but miraculously survives. When he wakes up in the hospital, he sees something strange: green numbers flashing on people’s backs. The numbers are visible only to him. As the day passes, the numbers decrease by one. Won-yeong learns that the back number indicates the days each person has left to live. If there’s a number 1 on one’s back, then today’s the day one will die.
Like this, Won-yeong becomes a “viewer.”

Everybody must face their last day of life. However, people live as if “the day” will never come since they don’t know when that will be. It is evident that everyone will face that at some point, but no one knows when that moment will be. However, Won-yeong now can tell when a person will die by looking at the number of their back. But ironically, he cannot know the number on his own back.

One day, Won-yeong sees a couple with the numbers on their backs flickering in red at a café. Won-yeong knows intuitively that there will be an accident that leads them to death, so he tries to save them from the immediate danger. Despite his efforts, they end up losing their lives in front of Won-yeong. Then a man without a back number approaches him. He warns Won-yeong not to intervene such as trying to prevent someone else’s death. Also he states that human intervention in life and death will cause worse trouble.
What choice will Won-yeong make over the fate in front of him?