Some reviewers complain that it’s hard to follow the plot – but isn’t this typical for film noir?
Besides, if you watch the movie again [or only the scenes that made no sense to you], you will notice that there is no puzzle. The whole thing about barrel-crossing the Niagara over the American side is just a subplot that is not important for the main plot. The main plot goes like that: we have several parties that want to obtain both parts of Tesla’s note. Some might want to destroy them, some are seeking for the knowledge of how they fit together and what they mean. In the end we see that Charlie has both parts in his lighter: one on the front, the other on the back. Of course Nikki Meeker has put it in there. Remember? She “stole” the lighter from him before. Now the light of the pinball machine and ta-da: Tesla’s potentially “world-changing” note in it’s entirety. Then the phone call for Charlie. But that’s part of The American Side II … which, of course, will never be realised.