The station is adjacent to a wormhole connecting Bajor to the distant Gamma Quadrant; the wormhole is home to powerful alien beings worshipped by the Bajorans as the godlike "Prophets", who have made Deep Space Nine's human captain Benjamin Sisko their "Emissary".
The later seasons of the series follow a war between the United Federation of Planets and the Dominion, an expansionist empire from the Gamma Quadrant ruled by the shapeshifting Changelings, which has already absorbed Cardassia.
Worf is missing in action after his ship is attacked by the Dominion, and Lt. Ezri Dax goes in search of him; both end up captured by enigmatic aliens known as the Breen.
Ezri is a member of the Trill species, symbiotically joined to the long-lived sluglike creature Dax; the previous host of Dax was Worf's late wife Jadzia, and the complex fact of Ezri's existence leads to complicated feelings between her and Worf.
Ezri Dax, feeling an obligation to Worf due to her memories of Jadzia, takes a runabout to the Badlands to search for him herself.