as far as lame plots go...

Oh snap!
That was good, I have to say.

The cake is the curse of this film.

And yes, I just can't get passed the fact that the plot for the entire film is set in motion by something so preposterous: that Neale should acquire a cake that conveniently has a secret microfilm buried inside. Not to mention, finding half a cake still intact after an explosion, and sitting on a brick wall. Yeeaaah, I don't think so.

First of all, why the *beep* would something so confidential be hidden in a CAKE?
Second, if you did hide it in a cake, why would you have it on display, and option it to be given away??

To find a convenient answer would be to assume that the microfilm was hidden without the lady (who gives it away) knowing it's inside. If so, why then did EVERYBODY go silent when Neale guessed the right weight, and then continued to leer at him when he collected the cake, and awkwardly stepped away? Doesn't that suggest that they know something he doesn't? If they know the cake has the microfilm, why the *beep* would they put the microfilm in the cake!!??
If they don't know about the microfilm, why are they acting like that?? NONE OF THIS MAKES SENSE!

Since the entire plot of the film hinges from this scene, I have to say that it is quite contrived, and lame, for a film to significantly depend on it.

The rest of the film is well made, however.
Ray Milland is once again, excellent in his role. He sells the performance of a man in danger and carries the picture. Lang does an appreciable job throughout, and the photography exemplifies some of the finest in the noir genre.

I'm curious to see how similar or different the Graham Greene novel is, but I believe the contrivances, for the most part, hold this film back from being something much greater, much more subtle.

Everyone knows that cakes are for rasps not microfilm.