Past Tense, Part I

En route to a symposium in San Francisco, Sisko, Dax and Bashir are lost during transport to Earth from the Defiant. The trio materializes in San Francisco, but in the year 2024. Apprehended by a pair of policemen, Sisko and Bashir are taken to a "Sanctuary District" — a gated area of the city where homeless, mentally ill and unemployed people are locked away from the rest of society.

Separated from the others, Dax is found by a prominent businessman named Chris Brynner, who takes her to his office and helps her obtain the ID card necessary to exist in this society. Dax doesn't reveal where she comes from, but tells him that her friends are missing. Meanwhile, in the Sanctuary District, Sisko and Bashir are taken to a processing center, where no records of their existence are found. Sisko learns the date and realizes that they have arrived just days before a crucial event in history, the Bell Riots, which occurred in the area of the city where he and Bashir are now confined.

Sisko tells Bashir that residents in the District will take hostages, but a man named Gabriel Bell, after whom the riots were named, will sacrifice his life to save them. Hundreds of innocent people will be killed during the riots, but Sisko and Bashir can do nothing, or this pivotal moment in history, which led to sweeping social reforms, could be altered forever. Later, Sisko and Bashir are given ration cards to obtain meals and sent out to find a place to sleep among the abandoned buildings.

On the Defiant, Kira, Odo and O'Brien realize their comrades were transported back in time, but cannot pinpoint the exact date, and have only a limited number of search attempts they can make. In the Sanctuary District, Sisko and Bashir meet Webb, a homeless but proud man who asks them to try and help the community, but they must refuse his request in order to avoid any contact that could change history. Later, Sisko and Bashir end up in a scuffle over Bashir's food card, during which a bystander who steps in to help is killed. Sisko is shocked to discover the dead man is Gabriel Bell.

When Kira attempts to contact Earth to discuss the situation, she learns that everything connected with Starfleet has vanished. Odo, O'Brien and Kira realize their missing crewmates have altered history, and that the Defiant is all that remains of the Federation. Back in the past, Chris Brynner does some research and learns that Jadzia's friends are among the thousands of people behind the walls of the Sanctuary District. Meanwhile, Sisko and Bashir realize that they must step in to undo the damage to history resulting from Bell's death. They arrive at the processing center just as the residents take hostages. As the riots begin, Sisko joins the aggressors, identifying himself as Gabriel Bell.