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.