Hello, Espionage keeps it's metadata stuff in the folder home folder/Library/Application Support/Espionage.
In Mountain Lion the Library subfolder in your home folder is hidden, so to access it, hold down the option (alt) key and select in Finders "Go" menu the "Library". Once you are in the Library you can navigate further into Application Support subfolder.
Rename the Espionage into Espionage.orig and log out and log back in. Launch Espionage and it should start with a default setup steps.
Here you can set the Master Password again.
But this will not bring you back the folders you protected before, if there are any.
So if you need to restore some data too, then let me know and I will write you further steps.
Rgds
Zsolt