Intro
The bulk of this story is made up of a written statement in which a former professor, Dr. Yu Tsun, tells of his experience working as a spy for the German army and of his reasons for getting involved in this line of work. More specifically, he describes an incident in which he found himself on the run from a British officer, his goal being to evade capture long enough to pass on a vital piece of information to Berlin.
With the clock ticking, Yu Tsun decides to pay a visit to "the only person capable of transmitting the message": a guy called Stephen Albert. The two spend some time shooting the breeze, but there's a job that needs to be done. The story ends with Yu Tsun shooting Albert and revealing that, by doing so, he has let the folks in Berlin know the next place they need to attack—a city called—wait for it—Albert.
This espionage plotline may be the narrative's driving force, but in the second half of the story, another hot topic raises its head. When Yu Tsun arrives at Stephen Albert's house, Albert assumes that he's visiting in order to see "the garden of forking paths." This rings a bell with his visitor, who soon realizes that this garden was created by one of his own ancestors, Ts'ui Pên.
But what is this garden? Thankfully, Albert thinks he's got it figured out: Ts'ui Pên had once said that he was planning to write a book and create a labyrinth. Everyone had imagined that these were two separate tasks, yet the book's unusual structure led Albert to realize that the book was the labyrinth.
Borges's story rocks for several reasons, its ending being one. (Did you call it? We sure didn't). We also need to remember that this is a text within a text, with the story starting out with a third-person narrator telling us that Yu Tsun's statement sheds new light on a historical event. Here, the narrator mentions Liddell Hart's History of World War I (a real book, by the way), which refers to a British attack that was planned for 24 July 1916 but postponed. This delay was supposedly due to heavy rain, yet the narrator points to Yu Tsun's statement as offering another explanation.
The narrator therefore presents this statement as a real, non-fiction text. That its first two pages are "missing" adds to this effect. Of course, this is all fiction, and the document Borges is writing doesn't actually exist, so the pages can't really be missing. But the point of all of this is to set up a framing device that guides our reading. Even then, Ts'ui Pên's book shows that there are always multiple possibilities.
Quote
To me, a barbarous Englishman, has been entrusted the revelation of this diaphanous mystery. After more than a hundred years, the details are irretrievable; but it is not hard to conjecture what happened. Ts'ui Pen must have said once: I am withdrawing to write a book.And another time: I am withdrawing to construct a labyrinth.Every one imagined two works; to no one did it occur that the book and the maze were one and the same thing. The Pavilion of the Limpid Solitude stood in the center of a garden that was perhaps intricate; that circumstance could have suggested to the heirs a physical labyrinth. Ts'ui Pên died; no one in the vast territories that were his came upon the labyrinth; the confusion of the novel suggested to me that it was the maze.
[…]
In all fictional works, each time a man is confronted with several alternatives, he chooses one and eliminates the others; in the fiction of Ts'ui Pên, he chooses--simultaneously--all of them. He creates, in this way, diverse futures, diverse times which themselves also proliferate and fork. Here, then, is the explanation of the novel's contradictions. Fang, let us say, has a secret; a stranger calls at his door; Fang resolves to kill him. Naturally, there are several possible outcomes: Fang can kill the intruder, the intruder can kill Fang, they both can escape, they both can die, and so forth. In the work of Ts'ui Pên, all possible outcomes occur; each one is the point of departure for other forkings. Sometimes, the paths of this labyrinth converge: for example, you arrive at this house, but in one of the possible pasts you are my enemy, in another, my friend.
Analysis
Though we haven't read the book ourselves, Albert tells us enough about it to give us a good idea of its non-linear structure. Usually, an author is faced with lots of possibilities when putting together a narrative, and he or she has to choose which one works best. This involves asking questions like "if such-and-such happens, then how does this character react?" or "if one character acts in this way, what effect does this have on the course of the narrative?"
Choosing between different options is the norm when crafting narratives, yet Ts'ui Pên asks why we should limit ourselves like this. His book consequently presents all the paths that its narrative can possibly take. Have you ever seen those old Choose Your Own Adventure books? Well, it's similar principle at work here.
While it's what a narratologist would call an experimental or unnatural narrative, Ts'ui Pên's "garden" doesn't just play around with narrative for effect. It's got more of a philosophical vibe, with an interest in time, fate, and inevitability. Rather than seeing time as a straightforward process, Ts'ui Pên interprets it as a labyrinth of endless possibilities. This also relates to the concept of history: the emphasis here is on history as something formed from multiple possibilities.
Ts'ui Pên's perspective may also affect our interpretation of the framing device that Borges uses at the start of the story: Liddell states that the British attack was postponed because of rain while Yu Tsun offers a different account. What we need to remember, though, is that neither Ts'ui Pên nor Borges are as concerned with finding out what really happened as they are with the concept of alternate universes and timelines. What's true in one timeline may be false in another, and vice-versa.
This interest in multiple possibilities is a major part of not only Ts'ui Pên's novel but also Borges's story itself—in fact, Albert's comments on the novel can't help but influence how we read this story. Also, we could say that Dr. Yu Tsun is writing his own story and one version of history through his actions here. Like Albert says, he and Yu Tsun are enemies in one possible past and friends in another. We're getting one version of events, but things can pan out differently.
It's kind of like that movie Sliding Doors: we're faced with several possibilities and are forced to follow one path or another. But we can wonder what would have happened if we'd chosen the other option.