Lu Sha resigns from her high-pressure job in foreign trade and moves to Haicheng to join her younger sister, Lu Wei, only to discover that she has been deceived. After Lu Wei is harmed in a car accident and left disabled, Lu Sha learns that the mysterious Jiang family, one of the city's wealthiest and most powerful families, is behind it all. Determined to uncover the truth, Lu Sha accepts help from Jiang Zhengbang when he reaches out to her, and the two begin a relationship based on a contract. What starts with mutual testing and a practical arrangement gradually grows into mutual respect and understanding. A relationship born from physical attraction and helping each other eventually leaves the proud Jiang Zhengbang completely and hopelessly in love.

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。