In a futuristic world where humans and mermaids coexist, the life of mild-mannered office worker Stephan is upended when he is suddenly proposed to by Chao — a princess from the mermaid kingdom. With no time to make sense of what's happening, Stephan soon finds himself living with the unpredictable, wholehearted Chao, and her sincere love begins to break down his emotional barriers.

Rud, an adventurer with the highest defence ever recorded, serves as a tank while exploring dangerous labyrinths in search of a treasure capable of curing his ailing sister. After being unjustly expelled from the Hero’s Party, he discovers the true nature of his hidden abilities and begins forging a new path alongside the companions who recognise his strength and selfless determination.