This is basically a repeat of a script that was done in season three. In that season three episode, 'The Great Necklace Caper', a chimp on Taratupa island steals a necklace off the desk of Binghamton, then Binghamton accuses McHale and his crew of the theft. This episode has the exact same plot except this time it is letters off Binghamton's desk instead of a necklace. Not the best to win fans when all they really did was rehash an old script.
What has happened is that a chimp has taken a top secret letter off of Binghamton's desk then made his way to McHale's camp also taking letters. Ensign Parker is going to follow some footsteps in tracking the letter thief back to his camp. In the meantime, Binghamton has set-up the thief by places some more letter on his desk. Sure enough the chimp returns and when Binghamton follows the chimp and finds him in company of Parker. Poor old Parker was just tracking footprints. Now Binghamton thinks the entire crew of the 73 are spies for the Germans.
How the producer can let this slip by before production is unknown. Perhaps they just wanted a episode in the can at any cost. All this was is the same script that has been modified to fit in Italy rather than the islands of the Pacific. Other than the actors, no one put much thought into this show. It was like saying' the viewers be darned' they will never know that they are seeing the same script. But thanks to IMDb the viewers are now informed. Giving this a two for the actor's sake, no other reason to give anything for a redone script.