The game is set in an Asian fantasy-inspired futuristic world which Red Candle Games describes as "taopunk", a combination of Taoism and cyberpunk, stating that “we had a clear goal of combining Taoism with cyberpunk, melding sci-fi elements with Eastern mythology.

[5] On the same day, a major patch was released, introducing a boss rush style gamemode called "battle memories" Five hundred years ago, the Tianhuo virus infected the Solarians, an anthropomorphic cat-like race.

Yi, the Tenth Sol and mastermind of the Eternal Cauldron Project, was attacked by his mentor and fellow Sol Eigong, but survived in a state of suspended animation due to a special connection with Fusang, a plant that produces vast amounts of energy that the Solarians harnessed for their technology.

Yi awakens several hundred years later in Peach-Blossom village, an area within New Kunlun where the Solarians had kidnapped humans to breed as livestock.

Over time, more residents come to the Pavilion: Shennong, a skeptical human who realizes the truth behind the village and brings Shuanshuan to the Pavilion, Chiyou, a former battle robot who gained sentience and became a scholar and merchant, Shanhai 9000, a reserved robot assistant who has its own personal agenda, and Kuafu, a Sol and close friend of Yi who decides to betray the Sols and upgrades Yi's weapons.

The game's combat was heavily inspired by Sekiro: Shadows Die Twice, Katana Zero, and Hollow Knight.

[7] Nic Reuben of Rock Paper Shotgun said he had mostly ignored platform games until Nine Sols, but its gameplay converted him into a fan of the genre.