To make money and pay off her debts, Li Xijian agrees to impersonate Lin Yinyin, the woman Jiang Yan—the blind and powerful boss of Hong Kong Island—has always loved. But during their time together, she gradually develops feelings for him. When the real Lin Yinyin returns, Li Xijian takes the money and leaves, only to discover that she is pregnant. Five years later, in order to pay for treatment for her daughter Li Li, who has leukemia, Li Xijian returns to Hong Kong Island. While looking for a job, she unexpectedly reunites with Jiang Yan, whose eyesight has already recovered.

月落和亞晨是一對雙胞胎兄妹，他們在一個與世隔絕的深山小村落裡出生，被稱為「分隔夜與晝的雙子」。 他們擁有獲得特殊力量的資格，一場圍繞著他們的雙使戰鬥也隨之展開。 　　雙使是一種成對的存在，被人類冠以「幽靈」、「怪物」、「怪獸」或「異形」等各種各樣的稱呼。 它們並非人人可見，各自擁有不同的外表、個性和能力。 每對雙使都有一個與之締結契約的主子，而指揮它們的主子被稱為「雙使師」。 　　許多大人在雙胞兄妹開始懂事前就暗中操縱，試圖奪取他們的力量，因此月落和亞晨在年幼時就被迫分離。 本作故事以兄妹倆的重逢展開序幕。