"Second Sight" consists of two main story lines that converge at the end of the episode. The first involves Commander Sisko's falling in love with Fenna, a visitor to Deep Space Nine. The second involves the effort to re-ignite a star by Professor Seyetik and the USS Prometheus (NCC-74913, not the same USS Prometheus as seen in of the Star Trek: Voyager episode "Message in a Bottle")
Seyetik, "one of the Federation's greatest minds", has the responsibility of re-igniting a dying star to make it suitable for supporting life on an orbiting planet. He has arrived at Deep Space Nine to prepare for his mission. Meanwhile, O'Brien and Dax assist him. O'Brien is preparing the Prometheus, the starship Seyetik will use, for travel at Warp 9.6âin case of emergency during the mission, the Prometheus would need to escape quickly from the vicinity of the star.
In the other story line, Sisko meets Fenna while he's standing on the floor above the Promenade staring at stars out a window. Fenna approaches him from behind and starts a conversation. After several moments, Sisko finds she has left him without a word. The two meet again in subsequent scenes during which they fall in love. Dax is apparently aware of their relationship; she comments to Sisko about it. It is the first spark of love he has felt since the death of his wife Jennifer four years earlier.
Seyetik invites Sisko, Kira, Dax, and a couple of other high-ranking personnel for dinner on board the Prometheus. During this scene, Seyetik introduces his wife, Nidell. To the audience, this introduction is dramatic because Nidell appears to be exactly the same person as Fenna. The next shot shows Sisko and Dax reacting to this surprising revelation.
After dinner, Sisko privately converses with Nidell, believing her to be the same person as Fenna. Sisko asks her such questions as why she didn't tell him earlier that she is married. Nidell, however, acts as if she'd never met him before. Indeed, she hadn't. The reason Nidell appears identical to Fenna is because Fenna is actually a psychological projection created by Nidell's mind. Sisko doesn't learn this until much later, however, which causes him great confusion.
Later, on board the Prometheus, Fenna appears to Sisko again. Immediately, he calls Dax to his location so that she may help investigate the nature of her mysterious being. This unusual response to her presence, of course, alarms Fenna. When Dax arrives, she determines that Fenna is nothing more than pure energy.
Meanwhile, Nidell is unconscious and dying. Seyetik recognizes Fenna, and explains to Sisko her psychoprojective nature. Nidell has projections such as Fenna during times of emotional stress. Sisko soon realizes why Nidell is distressed. She is no longer happy in her marriage, and since her people mate for life, she can't leave him no matter how much she might want to.
Dax calls Sisko to the command bridge, and informs him that Seyetik is in a shuttlepod on a course directly into the star. Sisko tries unsuccessfully to prevent him from his present course of action, but Seyetik is determined to set his wife free. As he approaches the surface of the sun, he shouts, "Let there be light!" The sun then re-ignites in an orange blaze. Nidell leaves the station to return to her homeworld, with no memory of Fenna's romance with Sisko.