"No One's a Mystery" by Elizabeth Tallent is about an 18-year-old girl who is having an affair with a married man. The short story focuses solely on a conversation the man and girl have about the future of their relationship.
"No One's a Mystery" begins with a man named Jack who is giving the girl he is having an affair with a diary for her 18th birthday. The couple is in Jack's truck and traveling down a highway somewhere in Wyoming. Minutes later, the man spots his wife driving toward them, so he pushes the girl's head down so his wife will not see her. Unaffected by the situation, the girl simply sings along to a song playing on the radio called "No One's a Mystery."
After they pass Jack's wife, the man and girl begin talking about what she will write in her diary. The girl assumes that in three years' time, she will be writing about her life with Jack, and by that point they will have gotten married and had children together. Jack disagrees, saying they will no longer be together in a matter of months. The story ends with no definitive answer as to what happens to their relationship. The reader is simply left to guess what happened next.