The series is based on the webtoon of the same name by Lee Yun-kyun and depicts a dystopian future where extreme air pollution, caused by the destruction of much of the world by a comet, forces survivors to rely on oxygen tanks and masks supplied by an elite of couriers.

Cho Ui-seok, known for films like 'Master' and 'Cold Eyes', signs both the direction and script of the series.

Kim Woo-bin plays '5-8', a legendary courier with exceptional combat skills, while Song Seung-heon plays Ryu Seok, the heir of the Cheonmyeong Group who became wealthy by controlling the sale of oxygen.

The cast features two actors for the role of Sa-wol at different ages: Kang You-seok plays the adult character and Choi Seung-hoon plays the young version of the refugee boy.

Similarly, the role of Seul-ah is split between Roh Yoon-seo in the adult version and Kim Ah-hyun, who plays the character as a child.

On the Rotten Tomatoes review aggregator, the series received 69% approval based on 11 reviews, with an average rating of 6.6/10.

The secret group known as 'Black Knights' is made up of eleven couriers, including '5-8', who work for the Cheonmyeong Group during the day and secretly help refugees at night, representing the only hope of overthrowing the oxygen-controlled world.