You know that movie scene in Indiana-style jungle exploration films where you stare at a rushing river until you see a primitive tribesman gangling about, sometimes on all fours, jumping from rock to rock and growling with the grace of an animal, then plunging its head into the water before pulling out with a fish in its teeth, and then it spots you and jumps across the rocks with superhuman speed before bowling over you and shrieking in your face?
These are parasites in certain rivers that turn ordinary normal humans into those guys. They were once a great race of water people- naiads- who were forced to retreat from the depths of the oceans and into the pristine streams after something something pollution(?). They fiercely guard their rivers in order to prevent their race from extinction. And this is the story of the Foundation contacting them and conducting diplomatic liaisons (and a few battles) with them.
