The special bodyguards of the private security company 'Tershio', Tsukasa, Shinrin, and Yasmin, are tasked with protecting a mysterious girl named Tonya. As they carry out their mission with the words "I will absolutely protect you" in mind, they are suddenly attacked by a man filled with an unusual killing intent, the Watchmaker. Faced with his extraordinary abilities, is there any chance of victory...? And finally, the mystery of 'Mermaid Lovers' is revealed――!!