On Khen Island lies a mysterious legend: alongside humans live another species—mermen. On land they appear no different, but once in the sea, their true form is revealed. Phurich, arriving from Bangkok to co-manage a bar with his friend Phana, quickly develops feelings for bartender Nava. But one night, after drinking, Phurich stumbles to the shore and falls into the water and catches a glimpse of what seems to be a merman.

After waking from a hangover, Phurich wonders if he truly saw a merman the night before. He mentions it to Phana, who brushes it off, yet later questions Nava privately to uncover the truth. Meanwhile, Phurich pursues Nava, sparking a rivalry with Phraphai. But rumors spread across the island of several missing mermen—will this looming danger disrupt the course of their feelings?

Phurich unexpectedly barges into Nava's home and continues to pursue him boldly, though Nava thinks he’s just a flirt. As they discuss the island's mermaid legends, Nava confesses that mermaids and humans cannot coexist, but Phurich believes that true love can overcome all boundaries. With Phurich's direct approach and Nava's secret identity, how will this forbidden relationship unfold?

Phurich wakes up hungover, and the atmosphere between him and Nava turns subtly intimate, though Nava remains distant. However, Phurich’s obvious affection doesn’t go unnoticed and his uncle sternly warns him that in the eyes of humans, mermen are nothing more than monsters or demons.

After a night of intimacy, Nava agrees to date Phurich, and the two officially become lovers. Phana seems to have feelings for employee Ping, yet he still gives his blessing to Phurich’s new relationship. However, Ping’s true identity gradually comes to light — he, too, is connected to the merfolk. Among the merfolk, a warning is whispered: humans and mermen should never fall in love. Can these two fragile relationships survive against fate?

As Ping struggles with his feelings for Phana and the truth about his identity, the rising wave of merman hunts puts Phurich and Nava’s forbidden love in grave danger.

On the eve of the full moon, Ping reveals his merman identity to Phana, receiving unwavering love and protection in return. But at the same moment, Phurich and Nawa’s bond faces the risk of rupture of a forbidden love.

Phurich heads into the merman village to bring Nawa back, but ends up poisoned by merman venom—his only salvation lies in a merman’s sacrificial blood. Can Phurich and Nawa overcome all odds and find their way back to each other?