I believe that “No One’s a Mystery” is a story about a young girl trying to accomplish an unachievable goal. The narrator is involved with an older man who is married and her goal is to “live happily ever after” with her knight in denim armor. However, he is still married and clearly has no intention of changing that fact, as is indicated by him attempting to hide her when his wife drives by. The majority of the plot revolves around what the narrator is going to write in her new diary over the next few years. However, she is struggling to open the book, which makes it even more obvious how irrelevant their own fantasies of the future are. No matter what, Jake is married.
They seem to live in a small town, and there is no room for redemption or forgiveness for infidelity in a small town. Even if Jake were to leave his wife in favor of the narrator, their location would not allow for the happily ever after for which the narrator is searching. She doesn’t get to write her own story, but Jake doesn’t have that power either.
In conclusion, the lock on the diary keeps her from writing her own story. But in reality, it is her location and circumstance that limit her future.