Joaquin Phoenix's character, Freddy, is an ex-soldier, drifter and an alcoholic. He ends up stowing away on the boat of Lancaster Dodd - Philip Seymour Hoffman - a self styled visionary who believes that he has found the solution to all of humanity's ills through a form of regression therapy that allows people to access their past lives. By the force of his personality he draws a following, and they become a kind of cult, hostile to the outside world, immune to self-criticism. We see, eventually, the "The Master", as he calls himself, is simply making his philosophy up as he goes along.
The exposé of how charismatic people can convince a group of followers that their nonsense is true was interesting, but could have been far more so. Apparently the story was partly based on the life story of L.Ron Hubbard, founder of Scientology, and there was a massive amount of potential to explore this further. Instead the film just drags on and on and on, and eventually runs out of steam, losing any sense of coherence near the end. I simply didn't care about anyone in this film, or anything that happened to them. That, to me, is its great failure.
The film thinks that it is profound and magical, but is simply flat. The cinematography is beautiful, and it looks great, but after two and a half hours I had simply stopped caring. Near the end Freddy beats up another of the master's critics, something he had done an hour of the film earlier. It was clear that the film had finally run out of ideas, and had given up.