The main premise is that as you head out of the galaxy, the more god-like you become. The inside is the 'unthinking depths'
There was an AI like enemy being, that computer archaeologists let escape, that wants to take over the universe.
One of the god-like beings puts the 'key' of the enemy's destruction into a human created from pieces of an old space battle.
The hero and the created human, who seem to be the only humans left in existence, go to a lower level, less evolved, civilized world, where a human explorer landed long ago who succumbed to the local forces or lizard people, to find the other part of the key and finally defeat the enemy AI by sending a wave of 'slowness', powered by their sun, making most of the galaxy not traversable by hyperdrive, effectively locking the enemy AI in 'space' with no means to travel.