Another close writing from Munro–by close, I'm using the "close reading" application, the same attention to diction and use of writing tools such as dialogue and character reflection to give information that a lesser writer might merely present as backstory.
The first person narrator/protagonist is recalling a past love and we discern her depth of involvement by how rather than what she tells us in the story. She is open yet protective, calling him "X" and mentioning that the letter is indeed a part of his name as well as being "expansive and secretive." She is staying at the home of a friend named Kay who goes through the same problems in relationships, yet has her own way of accepting them so she can accept herself.
Here is where Munro sets up plot by using character interaction, a bit of episodic device that gives us an interesting look into the two women through the eyes of one. If we pay close attention, we don't see the end as contrived or a planned twist, but a wonderful head-nodding moment, the reader not being taken by surprise, but rather noting that Munro's foreshadowing was wonderfully skilled and yet natural.