The race of sentient humanoids living beneath the waters of the planet Argo are unknown to the Federation prior of 2269. When the crew of the U.S.S. Enterprise investigates the world, its officers interact with the Aquans. They learn that the green-skinned people had once been air-breathers, but when the landmasses of the planet were submerged, they had adapted to underwater living thanks to a medically-induced mutation. They now live in Aqua-City, a place created from coral, deep in an underwater trench. There is a generation gap between older Aquans, who wish to stay below the water at all costs, and young Aquans, who are interested in rebuilding the surface world on what is left of the landmasses there.

The Aquans are governed by a Ruling Tribunal. Head of the group is Domar, the High Tribune, and while the group is mostly male, it has at least one female, a young woman named Rila. When Captain Kirk and Mr. Spock are caught "spying" on the Aquans in 2269, the Tribunal hears their case. There is also a Junior Tribunal, made up of younger members of the race. Aquan rules are called "ordainments."