15. Destiny

A team of Cardassian scientists arrive at Deep Space Nine to help build a relay that will allow communication through the Bajoran wormhole. A Bajoran priest, Vedek Yarka, tells the station's human commander Benjamin Sisko that an ancient prophecy predicts that this project will destroy the wormhole: "three vipers" (the Cardassians) will cause the "temple gates" (the wormhole) to be cast open and burned. Yarka is treated with skepticism, especially when it is learned that only two Cardassian scientists (Ulani and Gilora) are expected, not three. When they arrive, however, they explain that a third scientist will arrive later. This information makes Major Kira, Sisko's devout Bajoran second-in-command, begin to believe the prophecy. Yarka confronts Kira about her faith and her duty to help Sisko, whom Bajorans believe to be the Emissary of the Prophets.

When the third scientist, Dejar, arrives, she is greeted coldly by Ulani and Gilora. While Chief O'Brien and Gilora work on the station (complicated by the fact that Gilora misinterprets O'Brien's hostility towards her as a romantic overture), Sisko takes Ulani and Dejar through the wormhole in the Defiant to set up the relay. They discover a comet, which Kira believes to be the "sword of stars" mentioned in the prophecy. She discusses the prophecy with Sisko, but he chooses his role as a Starfleet officer over that of Emissary.

The relay is deployed and the test begins. A particular carrier wave causes the comet to change course and head towards the wormhole. If the comet enters the wormhole, a substance in its core, silithium, will cause the wormhole to be destroyed. O'Brien proposes modifying the Defiant' s phasers to be able to vaporize the comet. Sisko, despite starting to believe in the prophecy, goes ahead with the plan.

Instead of vaporizing it, the phasers fire a standard burst that breaks the comet into three fragments; O'Brien's modifications never came online. Gilora accuses Dejar of sabotaging the phasers, which turns out to be true: Dejar was sent by the Cardassians' intelligence agency, the Obsidian Order (which opposes collaboration between Cardassia and Bajor), to sabotage the mission. To prevent the silithium from destroying the wormhole, Sisko and Kira guide the fragments through with a shuttlecraft. Some of the silithium nevertheless leaks out and reacts with the wormhole; the reaction enables the communications link to be established. Kira and Sisko realize that the prophecy has come true: the three vipers were the comet fragments, and the silithium ignited the wormhole, "casting open the temple gates" to allow communication through them.