High in the mountains of Peru, archeologist Cody Westcott discovers an unusually inscribed stone tablet hidden in the ruins of an ancient village. Later, back home in Arizona, he interprets the inscription as a recipe for a potion that will give the drinker great vision. Cody succumbs to curiosity and concocts the potion, then uses himself as a guinea pig. But nothing happens. A sheepish Cody leaves the lab and heads for home, but as he steps out the door, his disappointment turns to shock and horror. The potion did work! It shifted his vision in a way that allows him to see the Interlopers -- grotesque creatures full of teeth and barbs and suckers that inhabit the trees, rocks, plants, and other natural objects around him. They are parasites from another dimension that infect any human being that comes into contact with their dwelling places. They fill their host with a dark mood that causes fear, anger, depression, and other negative emotions that the Interlopers feed upon.
When his wife is infected with these parasites, Cody's search for a cure leads him to the Society, a centuries-old organization dedicated to holding back the infestation and to thwarting the creatures' efforts to cause human misery. As the Interlopers prepare to trigger an event that will cause grief and pain on a global scale, Cody and the Society race to save the world from a threat it cannot even see.
I thoroughly enjoyed the imaginative and absorbing plot, believable characters, and engaging action of this book -- which I heartily recommend to any sci-fi fan. (Kim Corradini)