Identify speaker, plot and significance.
Her glance left me and sought the lighted top of the steps, where “Three o'clock in the Morning," a neat, sad little waltz of that year, was drifting out the open door. After all, in the very casualness of Gatsby’s party there were romantic possibilities totally absent from her world. What was it up there in the song that seemed to be calling her back inside? What would happen now in the dim, incalculable hours? Perhaps some unbelievable guest would arrive, a person infinitely rare and to be marvelled at, some authentically radiant young girl who with one fresh glance at Gatsby, one moment of magical encounter, would blot out those five years of unwavering devotion.