DS9 Season 3 Episode 11+12 Past Tense

Plot

Part I

When Commander Sisko, Dr. Bashir, and Jadzia Dax beam down to Earth from the Defiant, an accident occurs and they materialize in San Francisco in the year 2024.
Sisko and Bashir are found by a pair of police officers, who believe them to be vagrants and warn them to get off the streets. They are escorted to a "Sanctuary District", a walled-off ghetto that is used to contain the poor, the sick, the mentally disabled, and anyone else who cannot support themselves. Sisko realizes they have arrived just days before the "Bell Riots", a violent confrontation in the Sanctuary District that Sisko recalls as a watershed moment in human history. Dozens will be killed, including a man named Gabriel Bell, the leader of the demonstration. Bell will become a hero because of his self-sacrifice while protecting hostages, and public attitudes toward the disadvantaged will begin to change. Unable to find a building to sleep in, and unwilling to get involved with a nascent movement to air the residents' grievances for fear of altering history, Sisko and Bashir live in the street.
Meanwhile, Dax is found by a prominent businessman, Chris Brynner, who arranges accommodations for her. Later attending an affluent party as his guest, she realizes that Sisko and Bashir may have been taken into the Sanctuary District and persuades him to help confirm her suspicions.
Within the district, a fight breaks out when Sisko and Bashir resist attempts to take the ration cards they have been issued. A man who comes to their aid is killed, and Sisko and Bashir discover after the fact that he was Bell. Sisko quickly realizes that due to their presence, the course of history stands to be radically changed, since Bell is no longer present to safeguard the hostages in the historical narrative. The district residents begin to riot, and one group storms the processing center and takes the employees hostage. Sisko and Bashir return there, and Sisko assumes Bell's identity and takes leadership of the revolt in an effort to keep the hostages safe.
Back in the 24th century, the crew left on the Defiant, Major Kira, Odo, and Chief O'Brien, lose contact with Earth as all traces of the Federation suddenly vanish; Bell's death has radically altered the timeline. O'Brien calculates several possible time periods in which Dax, Sisko, and Bashir might have arrived, and Kira and O'Brien begin transporting to one period at a time in order to search for them.

Part II

Sisko tries to ensure that no one gets hurt during the standoff, while at the same time understanding that Bell must die at the end of it. He makes demands to the governor, insisting they be given airtime to express their grievances. He wants the Sanctuary Districts closed and their residents to be given opportunities to earn an honest living.
Dax watches news coverage of the riots from Brynner's office, knowing Sisko and Bashir are caught in the Sanctuary District and are in danger, and heads down to find them. She sneaks along an underground sewer line, but and is caught and delivered to the employment center to explain herself. Sisko and Bashir meet with her in secret to explain why they must stay until the crisis is over. She sneaks back out, certain that Brynner will be able to order a terminal activated at the processing center so that the leaders of the revolt can tell their stories and have them broadcast worldwide, which was the main force that turned public opinion against the Sanctuary Districts and led to their abolition.
After several fruitless attempts to locate their fellow officers, Kira and O'Brien finally transport themselves to the correct year and contact Dax. They stand by to rescue Sisko and Bashir if they can. Having rejected the hostage-takers' demands, the governor sends in a SWAT team to end the riots once and for all. The team kills many rioters, and Sisko is shot while protecting one of the hostages. Bashir finds the wound to be non-fatal, and the two are horrified to see the bodies littering the streets of the Sanctuary District as the riots subside. The two police officers who first confronted Sisko and Bashir agree to plant the pair's identification cards on two of the victims, and also to tell the truth about these events. All five officers beam back to the Defiant in the 24th century and find that the timeline has been fully restored, except for the fact that Bell's entry in the historical records now shows Sisko's picture instead of his own.