Answer: The narrator describes a man walking on a trail.
In "Ambush," O'Brien recalls his daughter Kathleen asking him whether he ever killed someone during the war. He tells her that he did not, but then reflects on this, and admits to himself that it is a lie. As he is reflecting, the narrator describes a man walking on a trail. This scene indicates that this is the beginning of a narrative within a frame story, and the reader can assume that O'Brien will now tell the story of how he killed someone.