Sirens can only have one undying love for one person in their whole life. Humans use this characteristic to emotionally bind the Sirens to human soldiers, forming the world-famous Siren Squad. Second Lieutenant Tang Yi’s twin brother, Tang Ya, was one of the captains of the Siren Squad, but died in a battle. Tang Yi was ordered to impersonate Tang Ya to control his brother’s Siren, Mercury, but he was found out just a few days after meeting. In order to protect the life of the Siren and continue to live his dream of being in the Siren Squad, Tang Yi and Mercury begin a happy(?) life in disguise as lovers.