Blackie, the mermaid maid of honor to the Sea Queen Three-Eyes, killed the Hunter and stole the Key to Hell. With his help, representatives of the Black Forces summoned a demon that only two magicians in the whole world could destroy. One of the two is Clotilde, a young detective of the secret organization "For Your Eyes Only". Clotilde is given the most difficult task in her career as a secret agent: she must win negotiations with the Sea Queen and return the Key. There are very few chances: the mermaids and the demon are doing everything so that the detective and her best friends do not live to see the end of the negotiations.