First introduced in the Prologue, the letter that Agatha leaves for Archie serves multiple functions within the novel, highlighting various conventions of the mystery genre. The letter serves, at various points, as a clue, a red herring, and an organizing logic that determines the arc of Archie as a suspect. Though some of the text of Agatha’s letter is disclosed in “The Beginning,” the “instructions” that this excerpt references are never explicitly laid out in their entirety. Leaving parts of the letter shrouded in mystery allows it to serve as an apparent clue to readers as they learn more and more about Agatha’s disappearance. Ultimately, it does serve as an actual clue to readers (when they learn that Agatha sent the letter and orchestrated her own vanishing), whereas to the police the letter becomes a “red herring,” a term used in mystery writing to refer to something that seems like a clue but is ultimately a misdirect.

The most complex function of the letter in the novel, however, is as a guide to Archie. By the end of the novel, the letter is known to instruct Archie to cooperate with the police and not to reveal outright that he knows that Agatha had anything to do with her own disappearance.