The letter that Kate shows to Si is the central symbol of the novel. It symbolizes the mystery that drives Si to join the time travel project, demanding that he be allowed to witness its mailing in 1882. Throughout the novel, Si’s thoughts return to the letter, which embodies the primary conflicts between Pickering and Carmody and between Si and Pickering. Like puzzle pieces, Si collects the clues first hinted at by the letter: the meeting between Carmody and Pickering in the park, the discussion of city hall Carrara, and the destruction of the World Building. As the pieces fall into place, Si realizes the ways he misinterpreted the letter and eventually understands the full story.

Even the heel print, though not a literal element of the letter, is a clue embodied by the letter as a symbol because the star and circle design first enters the narrative along with the letter in Kate’s story about her foster father. The curiosity engendered by the letter is the catalyst that pushes Si forward, even when he fears he might accidentally influence events in the future, and even Kate encourages Si’s questionable decisions because of it.