Eternal Poison begins in the Kingdom of Valdia where the demonic realm of Besek has suddenly appeared. This realm is home to demons called the Majin who have captured Valdia's Princess. The King of Valdia then issues a decree for her rescue which brings upon numerous adventurers of dissimilar intentions, the story of game is told from multiple perspectives of five key parties as they journey their way through the realm of Besek to rescue the princess.