The story is that he fired three shots from the Texas Book Depository building, the third bullet being the fatal bullet. In essence, that's it. The last half of the movie concentrates on the bullets and how the government is covering the whole thing up. The three collaborate and literally try and reconstruct what happened on that day. From what they put together, it's not hard to imagine that there is a lot of foul play going on in the investigation of the assassination.
Key witnesses tell that they have been harassed and that their statements have been changed.
Garrison is literally obsessed with the whole ordeal. Garrison proclaims that he is doing this so his kids can grow up in a world where freedom reigns and corruption is nonexistent, though the words are hollow as are his promises to his family to spend more time with them. Though the investigation process, the team comes across many people who all seem to know a little something about what happened. We, as people, are told that there were three shots fired from a building by Lee Harvey Oswald, and that the third bullet was the supposed fatal shot. People reported that a shot was fired at the grassy knoll in front of the motorcade.