The Land above and The Ocean below

Mary's father and older brother were killed mysteriously, their bodies washed ashore from the sea. Years later, Mary meets a girl on the street who looks lost. The girl then reveals a whole other world to Mary, where Sirens and Mermaids exist.

The girl also reveals her kind's intention to destroy the Land after an organisation on the Land started to capture underwater species and experiment on them, one of the former victims being one of their rulers.

Mary also meets new friends, and even the ruler who had been a victim of the organisation's experiments.

Before long, Mary starts to get attached to the new world she was exposed to and needs to decide if she wants to start a new life there together with her partner.