It retells the same Presidential assassination attempt several times, from a different point of view each time, adding details and asking new questions each time. It does this quite skilfully and manages to throw in a few plot twists in. Ultimately it is a bit pointless, though - with the motivations (and the plan) of several characters left unaddressed, I am left wondering more, and I'm not sure that the antagonists aren't just motiveless macguffins.
I suppose the kindest thing I can say is that I am very glad that films like this are made, and I will probably be buying the DVD (if only so that I can check if they cheated). I'd advise anyone wanting to see this to wait for the DVD release, too. I wonder if they will have an edit of it in chronological order, like the Memento DVD has.