Two friends, one of whom might be dead, an attractive stateless woman who was involved with the dead man, wet-cobbled streets, zither music - where have I seen all this before? In the far superior The Third Man, which appeared three years earlier.
As some other reviewers have pointed out, the writing of Diplomatic Courier leaves a LOT to be desired.
The 'McGuffin' would not have been a big deal, even when this turkey was first released.
On the plus side, crisp black-and-white photography, interesting settings that probably don't exist anymore, and a well-liked cast (sadly, misused).
Final note: this makes The Third Man look really, really good (good anyway, but you'll appreciate it even more.