'Critias' Summary

Essentially the story is about a good city and a city gone bad and the divinely arranged, therapeutic punishment of the bad city by its defeat at the hands of the good. 

— Warman Welliver 

According to Critias, in ancient times, the Earth was divided among the gods by allotment. The gods treated the humans in their districts much as shepherds treat sheep, tending and guiding them like nurselings and possessions. They did this not by force, but by persuasion. In those days, the areas which are now the islands of Greece were high hills covered in good soil. 

A number of great deluges came (including the global flood of Deucalion), and because no soil washed down from the mountains to replace the lost soil, the soil in that land was stripped away, causing much of the area to sink out of sight, and the islands that remained to become the "bones of a dead body." 

Athens, in those days, was very different. The land was rich and water was brought in from underground springs (which were later destroyed by earthquake). He describes the civilization of Athens at that time as ideal: pursuing all virtue, living in moderation, and excelling in their work.