In the year 2071, the entire South Korea is in a state of severe air pollution. After a comet swept through, it turned most of the Korean peninsula into a wasteland. Only 1% of the original population survived and were divided into different areas based on their social status. The ruling class lives in modern domes with abundant oxygen.

The common people live in desolate refugee camps, fighting for their every breath. At this time, the “Knights” appear, specialized delivery drivers, providing the people with oxygen sources and food and water to sustain life. One day, a legendary knight known as ‘5-8’ meets a boy named Sa-Wol, who wishes to become a Knight to avenge his murdered sister. The knight decides to train Sa-Wol so that he can survive in their dark and dangerous world.