An anonymous narrator introduces a document that will, he assures us, shed a little light on why a British offensive against the Germans had to be delayed by thirteen days. The document is a deposition (oral testimony given by a witness to be used in a trial) given by Dr. Yu Tsun. The first two pages are missing, so its narration begins abruptly.
Having learned that his cover as a German spy in London has been blown, Yu Tsun has only minutes to plan his next move. He must escape from Captain Richard Madden, the Irishman who has murdered his co-conspirator in espionage, and complete his mission by delivering the location of a secret cache of British weapons to his boss in Germany, whom he refers to as The Chief. He checks the contents of his pockets – revealing a revolver with only one bullet – locates the address of the one person capable of passing on his missive, and runs to catch a train to the suburbs.
Madden nearly catches up with Yu Tsun at the station, but he misses the train, filling Tsun with a sense of confidence that he will complete his mission successfully. At the Ashgrove stop, some creepy-looking children direct Tsun to the home of Dr. Stephen Albert. Tsun follows their instructions, finding himself following a continually forking road. He finally arrives at a pavilion, or summer house, from which he can hear the familiar sounds of Chinese music.
A man named Stephen Albert greets Yu Tsun, speaking Chinese, and invites him to see the "garden of forking paths." Tsun identifies himself as a descendent of the creator of that very garden. Dr. Albert tells Tsun the story of his ancestor, Ts'ui Pen, a former governor who abandoned his political position to write a novel and build a labyrinth, or maze. In the opinion of his descendents, Ts'ui Pen had failed on both accounts – the novel made no chronological sense, and the labyrinth was never found.
Stephen Albert, who has studied Ts'ui Pen's legacy for some time, explains to Yu Tsun that "the garden of forking paths" and the novel are one and the same and that the novel's seemingly incompatible storylines present the idea of the bifurcation, or splitting, of time, rather than space. In other words, whenever the characters come to a point at which more than one outcome is possible, both outcomes occur. This causes the narrative to branch out into multiple narrative universes, which then provide the scenarios for new bifurcations.
Seeing Captain Madden approach, Yu Tsun expresses his gratitude to Dr. Albert for resolving the mystery of Ts'ui Pen's garden, then shoots him in the back. Though Madden succeeds in arresting Yu Tsun, Tsun has succeeded in relaying his message – the secret weapons stash is in the city of Albert. Tsun reads about the bombing of Albert by the Germans in the British papers, the same papers in which The Chief was able to read the report of the murder of Dr. Albert by Yu Tsun.