Like a lot of books from Dick's "middle period", this contains a veritable hoard of good ideas that never really get developed. The Earth has been occupied by worm-like beings from Ganymede, Jupiter's largest moon. The occupation is opposed by a small resistance force comprised mostly of America's black population. The rest of the US seems to have reverted to something resembling the antebellum south, complete with the return of slavery. A cache of weapons is found that can create persistent and physical hallucinations, with a purported mega-weapon that can allegedly detach the Earth's entire population (and anyone telepathically linked with them - oh yeah, the worms are telepaths) from reality. One of the worm overlords goes native and becomes obsessed with 20th century fighter planes.

It's impossible to dislike a book that has this much stuff in it.