Although Louis XIV (Pierce Brosnan) has established his own empire, he has been struggling to find descendants who can inherit his throne. One day, he learned that the power of mermaids can give humans eternal life, so Louis XIV sent his capable general Yves (Benjamin Walker) to go on an expedition to the sea, and finally successfully brought back a mermaid (Fan Bingbing) and put her under house arrest. The ordinary girl Mary (Kaya Scodelario) was suddenly brought to the palace one day. In fact, her true identity is Louis XIV's illegitimate daughter. When Louis XIV encountered financial problems, he decided to use Mary to marry to solve his economic crisis. Mary met the mermaid, and a sincere friendship gradually developed between the two. At the same time, Mary also fell in love with Yves, who was responsible for guarding the mermaid.