In the undersea kingdom of fish and mermaids, Princess Era is a kind and beloved ruler who considers all sea creatures her friends and subjects. When her dear friend Bonito, a bonito fish, is caught by human fishermen and turned into a meal, Era defies warnings and transforms into a human to go ashore. She visits the restaurant that bought Bonito, intending only to mourn and pray for his soul. But a fellow diner convinces her that the only way to send a fish to heaven is to eat a piece of it. Reluctantly, Era takes a single bite—and is instantly overwhelmed by an irresistible craving for seafood. From that moment, she becomes addicted to eating the very creatures she once called friends.

Era's secret addiction throws her into a constant inner conflict: she feels deep guilt and self-loathing, yet she cannot resist the urge. She begins staying with a human family who run a seafood restaurant, using every excuse to order dish after dish. Each new chapter introduces a different sea creature—often with a heartfelt, dramatic backstory about its life and struggles—only for Era to eventually devour it in a beautifully prepared Japanese seafood dish. The manga blends dark comedy, genuine pathos, and surprisingly accurate marine biology, as Era's obsession grows while she struggles to keep her cannibalistic secret from her underwater friends.

Recurring characters include Sango, Era's loyal mermaid attendant; Urchiniko, a sea urchin friend; and various other sea creatures and humans. The narrative also follows the human family and their customers, adding subplots about their own lives. The story's arc moves from episodic indulgence to deeper questions about identity, desire, and the nature of friendship, culminating in a finale that forces Era to confront the full consequences of her addiction. Throughout, the manga includes actual recipes for the dishes featured, merging a gourmet element with its surreal premise.