The story is based on Thomas Pynchon's 2009 novel of the same name, which follows Los Angeles private detective Doc Sportello in 1969. Here's the official description of the novel.
"Part noir, part psychedelic romp, all Thomas Pynchon- private eye Doc Sportello surfaces, occasionally, out of a marijuana haze to watch the end of an era.
In this lively yarn, Thomas Pynchon, working in an unaccustomed genre that is at once exciting and accessible, provides a classic illustration of the principle that if you can remember the sixties, you weren't there.
It's been a while since Doc Sportello has seen his ex- girlfriend. Suddenly she shows up with a story about a plot to kidnap a billionaire land developer whom she just happens to be in love with. It's the tail end of the psychedelic sixties in L.A., and Doc knows that "love" is another of those words going around at the moment, like "trip" or "groovy," except that this one usually leads to trouble."
No details were given regarding Josh Brolin's role, but he joins a cast that includes Joaquin Phoenix, Reese Witherspoon, Owen Wilson, Martin Short, and Jena Malone. Paul Thomas Anderson is also producing alongside JoAnne Sellar and Daniel Lupi, with production scheduled to begin this summer.