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A long number sequence is provided in the one of his final flashbacks that revealed she had obtained her own death prediction from a Machine. Near the middle of the sequence the number 42 appears, a clue to its significance. This is the cipher, intentionally placed next to an emotionally evocative reveal in order to mask its incongruity.

The conversation in the woods provides near-blatant instruction for how to decode the text. His phrase, "Chapter One: How to decipher hidden meaning from simple sentence structure," instructs the reader to return to the first chapter of the story to begin decoding. Her reply, "Elementary, my dear," may call to mind early learning activities, such as letters and numbers. References to twos and pairs elsewhere in the text encourage the reader to break the number sequence up into two-digit chunks.

The final bit requires a leap but is easily verified as correct procedure shortly after it is begun. Take each two digit number in order, count that many letters into the sentences or lines contained in the first chapter of the story, noting as you progress the letter corresponding to each number.

The result should spell, "Lifeline by Heinlein," which is a reference to the first short story published by this author (technically, "Life-line," but incorporating hyphens would have raised the solution's complexity overmuch). That story, and the history of that story, provide additional context for ours.

Please note that this is the only explicit puzzle contained in the text.