This book is CRACKERS.
Like, I wasn't sure early on if I was going to finish it because it just seemed so peculiar. I did a lot of skimming. But there was enough humor to keep me going. The problem for me was that the humor was alternated with some passages of what felt like pretty heavy prose, and the tone felt very uneven.
I give you a sample of the hilarity, because when it was funny, it was very, very funny:

"Is that Dr. Warner?" cried Rosamund, bounding forward in a burst of memory, amusement, and distress. "Oh, I'm so sorry! Oh, do tell him it's all right!"
"Let's take hands and tell him," said Michael Moon. For indeed, while they were talking, another hansom cab had dashed up behind the one already waiting, and Dr. Herbert Warner, leaving a companion in the cab, had carefully deposited himself on the pavement. Now, when you are an eminent physician and are wired for by an heiress to come to a case of dangerous mania, and when, as you come in through the garden to the house, the heiress and her landlady and two of the gentlemen boarders join hands and dance round you in a ring, calling out, "It's all right! it's all right!" you are apt to be flustered and even displeased.

That mental picture kept me laughing for pages.

The main idea is that a boarding house full of dull people receives an unexpected visit from one Mr. Smith, a large man with grasshopper-like abilities who appears over the wall one day. He promptly throws everything into disarray and does all the things that no one's thought of doing since they were kids. But then there's an intended elopement, a run-in with the local doctor and criminal specialist, and a sudden distrust of Mr. Smith, who appears to have a very questionable record of violence and philandering.

The conclusion of the matter is... all kinds of batty, bonkers bananas.
But after I finished it, I keep revising my opinion a little and thinking, "You know, I think I liked it." Probably worth a reread now that I know what in the name of sanity it all means!
But don't read the Goodreads description. It spoils it all totally. It was more interesting to just come upon the answer by degrees.
UPDATE: I've just edited the Goodreads description to take out the spoilers, but it might take a while to show up. ( )

I found this novel much more difficult to follow (and less action-filled) than The Man Who Was Thursday. I got lost in the middle for several chapters, but I am glad I persevered because the ending made it into quite a powerful and interesting story.
Much of the book is taken up with a courtroom dialogue, which is why I got lost. The point of the courtroom is to determine if the antihero is guilty of several different crimes. This frames many different anecdotes about Innocent Smith; they are all tied together more and more as the story advances, and they are all tied to some central points of Chestertonian philosophy. ( )

What a strange little book this is! ( )

This comic novel encapsulates everything about Chesterton that appealed to Borges. It's hilariously funny and surreal, with the author's animus against pomposity in full force. Needless to say, the prose is excellent.

Chesterton claimed that "the normal was abnormal," that modern life forced people into paradoxical situations. For example, we are able to experience the glory of the highest pride and the awe of the meekest humility at the same time. Chesterton saw this as a manifestation of Original Sin and that a return to innocence was only possible for holy fools, such as this novel's central character. ( )

It might have been a good Father-Brown-like short story: the far-fetchedness of the plot excusable on account of the moral. Unnaturally, this novel just heaps more and more far-fetchedness while adding more morals only sparsely, and does it so in a way making, say, Walter Scott look like a succinct author in comparison. ( )