An Enterprise crew member named Lieutenant Daniel Kwan commits suicide seemingly at random while performing routine maintenance on the Enterprise's warp nacelle tube. He cryptically tells Riker "they were laughing at me" and "I know what I have to do", before jumping into the plasma discharge in the nacelle tube, vaporizing him. Captain Picard assigns Worf and Deanna Troi to investigate the circumstances surrounding Kwan's death.
Troi and Worf search Kwan's quarters and look at his personal logs, but it does not appear that he was in any way unhappy. On the contrary, he appears to be excited about his next several days and the prospect of spending time with his girlfriend, Ensign Calloway. When interviewed, Calloway suggests that the only thing Kwan seemed to be at all unhappy about was that he felt that his superior officer, Lieutenant Nara, felt threatened by him. Troi then returns to the scene of Kwan's death where she meets Lieutenant Nara, but Nara shows no noticeable dislike of Kwan at all.
While standing on the platform where Kwan killed himself, Troi is overcome by a sudden rush of emotions, which disorients and confuses her. Since Troi's empathic sense is overloaded, Dr. Crusher suggests that she rest for a few hours. Troi goes to her quarters to rest where she is soon visited by Worf and the two of them ponder the possibility that Troi was viewing an empathic "echo" left by Kwan, who it transpires also had empathetic abilities.
Later, Worf and Troi return to the nacelle tube where this time, Troi has a vision of a woman backing off and yelling "no" and an image of a menacing looking, red haired man. The two people fade, and Troi finds images of equipment with the markings of Utopia Planitia (where the Enterprise was constructed), and then sees the woman again laughing in a sinister manner with another man after the two of them are found kissing in a closet.
Worf snaps her back to the present, and the two of them leave immediately. It is then suggested that Troi was reliving an event from eight years ago, which was possibly experienced by Kwan since he himself worked on the construction of the Enterprise at Utopia Planitia. Troi and Worf search personnel files for the people Troi saw in her vision and find the red haired man to be Lieutenant Walter Pierce, currently serving on the Enterprise as an engineering officer. They also find out that Pierce happened to be Kwan's supervisor at Utopia Planitia, but when they talk to him he claims to have no knowledge of anything out of the ordinary happening in the nacelle tube. Having returned to her quarters, Troi notes to Worf that she could not read Pierce at all, meaning that he too must be partially telepathic. Worf says good night to Troi and begins to leave, but instead Troi and Worf spontaneously kiss and it is intimated they spend the night together.
The next morning, Crusher administers a neural inhibitor to block Troi's empathetic sense, allowing her to revisit the nacelle tube without experiencing further hallucinations. Worf arrives shortly after, however he is indifferent towards Troi and tells her that he is unable to accompany her to the nacelle tube, instead assisting Ensign Calloway.
Troi, Data and Geordi look at a plasma conduit that Kwan repaired shortly before he killed himself, which Nara explains had probably never been previously opened. Despite her inhibitor, Troi sees another vision of Pierce and the unknown woman. She is positive that something is behind the wall and Geordi discovers a human skeleton embedded in the ship.
Calloway searches through personnel files and finds the skeleton to be that of Ensign Marla Finn, who is revealed to be the woman that Troi has seen in her visions. It is discovered that Kwan arrived at Utopia Planitia well after Finn had disappeared, and Troi comes to the conclusion that the image of Pierce she saw was a reflection, and that the vision was taking place from his perspective. She and Worf go to arrest Pierce, but Troi starts to wonder again when Worf seems to be behaving very attentively towards Calloway.
Troi asks Worf if he is unhappy about their relationship and Worf is adamant that he is not. He suggests that Troi is feeling exasperated and she agrees, returning to her quarters while Worf goes to accost Pierce. However, Pierce soon enters Troi's quarters, alone, and gives her a long and unnerving stare. Troi immediately calls security and asks them to hold Pierce in his quarters, but as he is being escorted away Pierce says that Worf did not interrogate him, as he "had to go somewhere". Confused, Troi asks the computer to locate Worf and it is revealed that he is in Calloway's quarters. Troi rushes there and finds Worf and Calloway hugging each other romantically. The two of them turn and laugh at her in the same way that Finn and the other man did in the vision.
In a fit of rage, Troi picks up a phaser and kills Worf. Shocked and disturbed, she runs about the ship and again encounters Pierce who tells her that she knows what to do. Troi agrees and rushes to the nacelle tube to kill herself in exactly the same way that Kwan did. However, she is stopped by a very much alive Worf.
It transpires that only a few seconds have passed since Troi first experienced her hallucination; she was imagining everything that happened afterwards. What seemed like a few days was only a couple of short seconds. It is revealed that Walter Pierce, Marla Finn and the man with Finn were all killed at Utopia Planitia years ago in a plasma discharge. Troi hypothesizes that Pierce and Finn were romantically involved and that Pierce killed Finn, the other man and himself in much the same circumstances as Troi's vision. Because he was partially telepathic, Pierce left behind a residue when he killed himself, causing any other telepathic person who worked on the nacelle tube to succumb to an altered vision of his own experience. Scans of the plasma conduit in question confirm that this is the case.
All is well, but after Worf asks Troi why she was so surprised to see him alive she shares an awkward moment with him, similar to their encounter at the end of another seventh-season episode, "Parallels".