A mystery with no resolution, the whole thing left you hanging. It depended on the conversation between Sherlock and Watson than on any logic that could be discovered.
Holmes is asked to travel to an Abbey where two monks have fallen ill. He agrees and on arrival is met by the Abbot who explains the circumstance of the two monks being taken ill. He and Watson examine the manuscript that seems to be the source of the problem. From there they discover an issue with the book. From there they reach a conclusion that they share with the Abbot.