In order to find out the cause of water pollution in the Demon Realm, Leo and Mernes head off to investigate Lake Amia and its resident water spirit, Undine. But one thing stands in their way-the legendary "Demon King Slayer," who swears to protect Undine at all costs! Just who is this mysterious foe?! Meanwhile, Demon Queen Echidna puts her plan into motion, donning a perfect disguise to confer with Leo about her troubles.