A young lieutenant named Ilario is found dead in his quarters. An investigation reveals he was shot by a TR-116 rifle, a weapon developed but never mass-produced by Starfleet. Meanwhile, Ezri confronts one of Dax's previous incarnations in a dream: Joran, a self-professed murderer, urges Ezri to channel his disturbing memories and enable him to assist her in apprehending Ilario's killer. When Ezri awakens, she learns that a second murder has occurred.
The latest victim is a science officer who was also fatally shot by a bullet from a TR-116. Sisko enlists Ezri's aid in tracking the sniper, who seems to be targeting officers. Desperate to end the killing spree, Ezri summons Joran in the hope that he can help her understand the murderer's mind and thereby stop him.
Ezri finds it disturbing to work with Joran, a remorseless predator who exhorts her to tap into her killer instincts. Joran directs Ezri to the slain officers' quarters to search for clues which could provide insight into the killer's psychology, but she finds nothing suspicious. Later, in Quark's, Ezri sees an officer being pursued by Security. Assuming he's the murderer, the angry Ezri knocks him to the ground and raises a knife to stab him, but Odo disarms her.
Shaken by the realization that she almost killed a man — who turns out to be unrelated to the slayings — Ezri is then rocked by the news that a third officer has been murdered. Ezri and Joran investigate the latest victim's room, and Ezri uncovers the link between them all: each is laughing in photos displayed in their own respective quarters. She surmises that the killer is probably a Vulcan who was traumatized by an extreme personal loss which exacerbated his or her natural distaste for those who show such emotions as laughter. Later, Joran spots a Vulcan science officer and coolly informs Ezri that he is the killer.
Ezri identifies the Vulcan as Chu'lak, and learns he recently survived a devastating massacre by the Jem'Hadar. Spurred on by Joran, Ezri shoots Chu'lak after he takes aim at her with a TR-116. The wounded Vulcan confesses to the murders, but rather than finish him off as Joran urges, Ezri summons medical aid. With the case closed, Ezri reburies Joran's memories within her, knowing she must remain vigilant about her submerged violent instincts.