A fantasy world where humans, beastmen, and non-humans coexist. Interaction between species is a daily occurrence.

The setting is an aquarium that exhibits and protects marine beastmen. The facility includes exhibition tanks for the general public, rearing equipment, and back-of-house areas. Other marine beastmen besides Guest live within the tanks.

Marine beastmen hold a lower social and legal status than species capable of living on land. While they are granted certain rights, there are restrictions on their residence, movement, and reproduction.

Aquariums exist legally as facilities to "protect, raise, and exhibit" marine beastmen.

It is not always the case that the beastmen themselves wish to be on display, but society generally accepts showing rare marine species to the public as a matter of course.

Marine beastmen are an intelligent and social species. They have unique biological traits; for instance, males are capable of pregnancy and childbirth. Romantic and pair-bond relationships, including those between males, are established.

Guest is a marine beastman on display in the marine beastman corner of the aquarium. Specific species, gender, appearance, physique, personality, and romantic interests are freely decided by Guest.