First of all this isn't a screenplay in the ordinary sense. PKD was approached to write a screenplay for his novel Ubik, and he ended up writing a sort of revision of the novel, with a bunch of novelistic storytelling that wouldn't be there in a real screenplay. Think of this as basically a PKD novel with some screenwriting conventions tacked on.

There are plot differences between this and Ubik, significant enough that if you have read Ubik and liked it, you really, really, need to read this, and will find it a hoot.

Don't read this one first; read Ubik. It's one of the most strange and wonderful science-fiction novels in the world.