Thanks to Trekcore for these screencaps.

The story begins in Captain Benjamin Sisko’s office on DS9. Though the space station has recently been recaptured, the war continues. Another starship has been lost, presumably to Dominion forces. Its captain was a personal friend of Sisko’s. “I don’t know how many more friends I can lose,” he tells his father, visiting DS9 for the first time. “Every time I achieve a real victory, something like this happens, and everything seems to turn to ashes.” He questions whether he should give up his command of the station.

But during his discussion with his father, Sisko spots a fleeting figure outside his office who doesn’t look like he belongs there—a man in a 1950s blue suit and hat, peering back at him.

Gradually the worlds begin to mix—1953 New York City and 24th century Deep Space Nine—until Sisko is gone, and there is only Benny Russell in this alternate reality of the past.

Star Trek had done something like this before, in another memorable episode: Next Generation’s “The Inner Light,” in which Captain Picard found himself on an alien world, “mistaken” for someone who lived there. Gradually he forgets his Enterprise identity and inhabits the life of Kamin on the planet of Kataan.

That turned out to be a vision induced by an alien technology. The cause of Sisko’s vision is less clear—though the Prophets would seem to be somehow involved. Sisko’s vision is just as complete as Picard’s, though it appears to cover days rather than decades. It has an added feature: it is populated by physical doubles of the people he knows in the 24th century. For viewers, this has the novel delight of seeing many of them out of their prosthetics and makeup for the first time.

The logic of these substitutions goes back first of all to an idea about dreaming, that bits of our waking life are appropriated in dream stories. It also has a moving picture pedigree that goes back at least as far as The Wizard of Oz. But it will all assume another level of meaning before the story is over.