“Past Tense: Part 1” is episode eleven of season three of Star Trek: Deep Space Nine.
The Defiant is headed to Earth so that Starfleet can be briefed on the current situation in the Gamma Quadrant. There’s a formal dinner that needs going to, but Major Kira, Odo and Chief O’Brien manage to back out – the first two because they are not in Starfleet, the latter because he’s enlisted (and he says that’s why he’s stayed that way).
When Sisko, Dax and Bashir attempt to beam down, something goes wrong (a transporter accident, how unusual). Although O’Brien’s logs show that the three successfully beamed somewhere, they never arrived at Starfleet.
Instead, the three have gone back to the year 2024. Back when this episode was made, that was about three decades in the future. Now, it’s a lot, lot closer. The situation in San Francisco is rather different. Sisko and Bashir get scooped up and put in a Sanctuary, because they had no ID. Dax was rather luckier and got helped by a friendly stranger.
Sisko realises that they have arrived only days before a famous riot in the Sanctuary they are in, one that had the effect of changing things, all because of one man.
The story continues in the next episode, “Past Tense: Part 2”.