Mermaid Sera saves a drowning human, Zane, but after a one-night stand, she loses her silver hair, fishtail, and most importantly, her heart scale. Forced ashore to retrieve it, she lands a secretary job at Zane's company—only to find her best friend Lucy has already impersonated her as his rescuer. Sera navigates workplace traps, clashes with Lucy, and slowly falls for Zane. Her childhood friend Caspian reveals that he, not Lucy, was the one who saved her years ago. Driven mad, Lucy summons a tsunami. Zane drinks a potion to save Sera, merging the heart scale with his own heart. Sera regains her mermaid form and calls sea creatures to rescue everyone. Lucy is cast into the abyss by the Sea God. Because her heart scale once resided in a human, Sera gains the ability to live both on land and in sea, ultimately breaking the boundary between species and staying with Zane forever.
