The movie follows Bill Pullman (Lost Highway) as a prominent scientist studying brains. He is requested by his boss to study the brain of a potential madman-genius suspected of murder. He accepts reluctantly. After a visit to the suspect, he is victim of a car accident and this is when the movie starts really. What you see then might be hard to follow and it's even harder to explain...
Accident. Fade to black. Pullman wakes up and try to solve the murder. Fade to black. Pullman wakes up and he is the murderer. Fade to black. Pullman wakes up and he is a victim. Fade to black...and it keeps going until the final twist.
The movie investigate what happens in your mind when you are in a coma following a severe head injury: what is real? What is memory? What is dream? What is nightmare? All experiences are mixed and while Pullman lives the events you find yourself trying to understand them... What happened to him? What did he really do? What is his real relation to the suspect? Which family are we talking about?
Granted the movie is not of the "best quality" but it's not important. First, the production and realisation are much better than, for example, the homonymous Brain Dead from Peter Jackson. Second, the movie's interest resides in you wondering, analysing, interpreting and finally understanding what is really going on.
If you want a comparison, you can compare this movie to Solaris: it explores something that you might or not believe in (respectively for Brain Dead and Solaris: consciousness and cognition during a coma or the resurgence of long-dead loved ones), leaving you time for you to think during the scenes. If you watch movies without a desire to learn and think, just do not buy it. If you have such envies, it's definitely for you.