Complex sentence: In the present, Allie becomes briefly lucid and remembers that the story Duke is reading is the story of how they met, as Duke tells her of a younger Allie appearing at Noah's doorstep, having left Lon at the hotel.
Split into simpler sentences: In the present, Allie becomes briefly lucid and remembers that the story Duke is reading is the story of how they met. Young Allie appears at Noah's doorstep, having left Lon at the hotel.