After a night of heavy drinking, Ezri Dax accompanies Ilario, a fellow DS9 crewmember, back to his quarters. The next morning Ilario is found dead there.
Julian Bashir determines that Ilario was killed by a tritanium bullet fired at close range. The weapon is determined to be a TR-116, a prototype designed by Starfleet for use in environments where a phaser would be ineffective. But Odo is puzzled, because that kind of weapon usually leaves powder marks on the victim when fired at close range.
Joran Dax, the most violent of the former Dax hosts, surfaces from Ezri's subconscious. Joran tells Ezri he can help her find Ilario's killer. Ezri summons him to the foreground of her consciousness with a Trill ritual.
Demonstrating on a melon, Miles O'Brien shows Odo and Dax how the killer gave the bullet a close range trajectory without being in the same room as the victimâa micro-sized transporter had been fitted to the weapon, enabling it to shoot through walls or any other obstacle.
A 37-year-old female human, and later a male Bolian, both turn up dead with tritanium bullet wounds. The only pattern appears to be that the killer is targeting Starfleet officers, until Ezri notices that each of three murder victims had photographs in their quarters that show people laughing.
With Joran's help, Ezri narrows the list of suspects down to 48 Vulcans. Riding in a turbolift with Chu'lak, one of the suspects, Joran becomes convinced Chu'lak is the killer. Ezri uses a TR-116 to shoot Chu'lak at the same time Chu'lak shoots at her. The bullet for Ezri misses by a narrow margin, but the bullet for Chu'lak hits its target.
Chu'lak is revealed to have been a crewmember on the Starfleet vessel USS Grissom, which experienced severe casualties during the Dominion War. Chu'lak was one of the ship's few surviving crewmembers, and the emotional shock of the experience drove him to target any Starfleet officer showing happiness. While being questioned, Chu'lak attempts to justify his actions by saying "Because logic demanded it." As Joran demands that Ezri finish him off, she does not and calls a medical team.