The planet Argo was once a land planet, but by the year 2269, its surface is almost completely covered by water. The change was caused by violent seismic disturbances. On stardate 5499.9, an aquashuttle is launched from the U.S.S. Enterprise, carrying a team which intends to study the effects of the quakes and other phenomena on the surface; another Federation planet will soon be facing the same kind of immense geographical changes. The planet has an orange-hued atmosphere, and emerald-green water. Although sensor scans show no life-forms other than fish, the Enterprise crew soon learns that the civilization of Aquans is very much alive.