Nijuuei is a visual novel adventure game set in Japan in the year 1640 centering around Futatsu-kage Sougen, a swordsman who, for reasons unknown to him, holds host to an antagonistic and violent sentient curse he dubs "Nijuuei" that causes him to cast two shadows and wishes to possess him.

The game begins as Sougen travels to the strange and cursed island of Awahomurajima in hopes of finding a way to dispel the curse placed upon him.

Gameplay consists of the player making choices, some of which being timed in battle scenes, and following different story routes to the conclusion.

Nijuuei features the "PIS" (Phrase Interaction System) where some words or phrases are highlighted and may be selected to view an explanation of the highlighted phrase or an expansion upon a related topic.