Brilliantly, the franchise decided to jump from the very heavy, convoluted and intense Emissary which revolved around Ben Sisko and the precarious situation which would center on him for the next seven years directly into a no-less-serious but very different drama. More importantly, this is the episode which first unleashed the great Andy Robinson into his very beautifully developed recurring character Elim Garak - the most popular Cardassian in Star Trek history and perhaps the most sympathetic alien (besides Spock) in the canon. Garak is, of course, introduced by way of his relationship with Dr. Bashir (Alex Siddig, then credited as Siddig El Faddil) and throughout the series Siddig's best episodes will most often be those featuring Garak.
Garak is either a left-behind Cardassian tailor or a Cardassian spy or both, and he is particularly concerned about the arrival of a Bejoran "Terrorist" (or freedom fighter, depending on your perspective) who has been running from Cardassian authorities and is seeking asylum. Kira,of course, knows Tahna Los, the suspected terrorist, and pleads with Sisko to allow him to stay.
As the story develops, the powerful ethnic conflict between Bejorans and Cardassians becomes a central focus. This will be well-exploited throughout all seven of DS9's seasons. And Garak's contribution to the depth and range of what it is to be a Cardassian ensured that the Cardies could never become just another franchise stereotype disguised by a forehead prosthetic. Finally, Past Prologue allows some breathing room for Avery Brooks to settle into his role as Ben Sisko, and includes some important developments in his relationships to Kira, Odo, and the Cardassian military.
Acting highlights - Robinson, Visitor, Siddig, Brooks and Auberjonois are all terrific in Past Prologue.
As usual, the episode is economically directly and scripted, well-shot and thoroughly entertaining from start to finish.