Alfred Hitchcock popularized the term "MacGuffin" as being "what the spies are after, but the audience doesn't care." In this movie, the MacGuffin is Clause 27 of a treaty between two countries. It is a secret clause, so secret in fact that it is only known to the two people who signed the treaty, because it was never written down.
Now, whether it is a treaty, a contract, or any other kind of agreement, the whole point in writing it down and having people sign it is so that there is no question as to what was agreed to. Anything not written down can be denied later, especially since there are no witnesses to this oral agreement between the two signatories. I guess we are to assume that the two diplomats trust each other so much that an oral agreement and a handshake will suffice.
This raises the question as to how anyone other than the two signatories knows of the existence of Clause 27. The spies know about it, as does Scott ffolliott, so I guess the two signatories must have announced that they had signed a treaty with an unwritten clause. It seems to me it would have been better to keep not only the content of the clause a secret, but its existence as well.
One of the signatories is Van Meer. To find out what is in Clause 27, the spies kidnap Van Meer with the idea of torturing him until he talks. But to keep the world from knowing that Van Meer has been kidnapped, they get a man who looks like Van Meer to take his place so he can be assassinated. Presumably, the impostor did not know about that part of the plan.
If the world thinks Van Meer has been assassinated, then that means that as far as everyone else is concerned, only one person knows what is in Clause 27. Van Meer might have trusted this other fellow, but can we expect the country he represented to honor a secret clause whose content is known only to the diplomat of the other country and take his word for it? So with Van Meer's faked assassination, it would seem that the clause has just become worthless. Or maybe the spies were planning on releasing Van Meer after he spilled his guts saying, "Fooled you. Van Meer is alive after all, but you still have to honor the secret clause that we now know about."
Moving right along, if I had been Van Meer and the spies started torturing me to tell what was in Clause 27, I would have just made up something. After all, it's a secret, so how would the spies have known the difference?
But enough of this. The point of the MacGuffin, as noted above, is to give the spies something to pursue that the audience is not expected to care about. But that's just the problem. Maybe we are not supposed to care about what the MacGuffin is, but we sure are supposed to care about what makes the MacGuffin important. Over and over again, we are continually being prodded with a preachy message about the need to take a strong stand against Germany. In short, this is another of Hitchcock's propaganda films, the first one being "The Lady Vanishes" (1938). This is why Stephen Fisher, who is the leader of the Universal Peace Party, a pacifist organization, actually turns out to be a Nazi spy. You can't trust those peaceniks. The problem is not with the message per se, but with the enervating effect of propaganda. Who wants to watch a movie and be lectured to? Of course, there are enough good scenes in the movie, especially the one in the windmill, to make the movie enjoyable overall, but it is somewhat spoiled by the warmongering.
Fisher has a daughter named Carol who is the love interest of the title character, Johnny Jones, who is forced to take on the pseudonym of Huntley Haverstock. He agrees to get Carol to go to the country with him so that ffolliott can make Fisher think his daughter has been kidnapped and thus arrange a prisoner exchange for Van Meer. The pretense is that Haverstock needs to hide from the spies, who are trying to kill him, because he knows who they are. When Carol and Haverstock get to Cambridge, they get a room at a hotel.
Ooh la la. One room for the two of them! Even if it is just for the afternoon, it sounds very cozy, and Carol seems just fine with it. But then ffolliott calls Haverstock and tells him he needs more time to talk to Fisher, and so Haverstock will need to keep Carol there overnight. Haverstock agrees and makes an arrangement with the hotel for another room for Carol. Carol overhears this and is appalled.
Now, I know that things back then were different regarding sex, but I cannot figure this one out. The very fact that Haverstock is getting a separate room for her indicates that his intentions are honorable. But the woman who was just fine having one room for the afternoon is outraged that he would get a separate room for her for the night. I guess she thought that the second room was just for appearances, and that he was planning on slipping into her room later that evening, just the sort of thing a man might have on his mind while hiding from spies who want to assassinate him. Since they were hiding from the spies, she should have figured that something had come up necessitating a longer stay. The reasonable thing for her to do was go up to him and say, "Why are you getting another room for me so we can stay overnight?" But not much else in this movie makes sense, so there is no reason for this scene to be any different.