In the world of the Beautiful Girl Task Force Sailor Actis, Sailor Princess and Sailor Mermaid are very close, always sticking together whether at school, studying, walking home, or fighting monsters. However, Devil King threatens their bond by sending a malicious monster after Sailor Princess. Sailor Mermaid rushes to save her and defeats the creature, but a minor disagreement arises between them, creating a rift in their friendship. Seizing this opportunity, Devil King takes action—Gentle Mask, once a gentleman and the object of admiration for the Sailor Warriors, has been brainwashed and transformed into a monster. He now targets the lonely and vulnerable Sailor Mermaid in a long-anticipated assault.