There is a crocodile which has been alive since the Carboniferous era. These days it mostly sleeps and dreams about the past. Behind Gently Smiling Jaws is a setting which takes place in those dreams.
The dream world was discovered hundreds of thousands of years ago by the Naacals. These are a race of human beings who lived on a continent called Mu (which has now sunk). Mu stretched from Mexico in the East all the way across the Pacific to India in the West, and encompassed Indonesia and New Guinea. The Naacals were the ancestors of the Mayas and also of the first civilizations to take root in India. They are now gone, except for those who migrated into the crocodile's dream world, which they believed to be a paradise. They founded a city in the middle of it, in a vast ocean, and they live there still: the PCs are Naacals.
The dream world is made of mythago stuff which will warp and transmute under the influence of powerful psyches. The Naacals discovered a way to avoid this, though the method is now forgotten. But seven powerful interlopers have subsequently also independently entered the crocodile's mind, and their puissant mental energies have fundamentally changed the character of entire regions of the dream world.
There are seven of these regions, each being a volume of the book.
Vol 1 concerns The Infinite City. This is the crocodile's memory of an ancient city which pre-dated even the Naacals - a bit like Atlantis except it existed long before even Atlantis did. It was a city of canals (somewhat like Venice) and built of warm-coloured sandstone (pink, orange, etc.). The crocodile remembers it as a kind of jumbled up Escher-like vision of crazy architecture broken up into chunks by a huge network of canals. It is now home to a Portuguese conquistador called Jorge de Menezez who has created a city-state in the middle of it. He dreams of using this as a base to raise an army by which to go back to the real world and conquer it and become an emperor. But because his character is militarist and expansionist, the entire Infinite City is like that now as well, so it exists in a state of continual warfare between bitter and implacable rival factions.