In a fantasy realm threatened by the Demon Lord's revival, powered by a mysterious red moon that bolsters demonic forces, the courageous warrior Leona battles invading monsters to protect humanity. Overwhelmed by sheer numbers, she is rescued by Hain, a human scholar secretly researching countermeasures against the empowered demons. His solution shocks her: to gain resistance to the red moon's corrupting influence, Leona must endure tentacle violation, impregnation, and oviposition (egg-laying) by demonic appendages, which paradoxically boosts her combat prowess through the resulting offspring or enhancements. Though resentful of the deception and the humiliating process, Leona accepts it as necessary for victory. Joined by allies like the rescued Akris and others encountered on their quest, the group travels to confront the Demon Lord's Four Heavenly Kings, facing seduction, enslavement, and tentacle domination in various kingdoms. Each encounter tests their resolve, blending heroic fantasy adventure with explicit erotic corruption. As Leona and her companions grow stronger through repeated "rituals," they push toward the final confrontation, exploring themes of sacrifice, power, and forbidden pleasure in this milestone Venus Blood anime adaptation.

Based on the erotic game by Dual Tail.