A killer in the station targets Starfleet officers.
A young officer, Ilario, is found shot dead in his quarters with a bullet from a TR-116, a discontinued prototype rifle. O'Brien works out that the killer fitted the rifle with a tiny transporter and sensor, letting the shots be fired from anywhere on the station.
As more officers die, Ezri summons the memories of Joran, a past Dax host who was a murderer, to help her profile the killer. Noticing that every victim kept a photo of people laughing, she concludes the killer is a Vulcan traumatized by the war and identifies Chu'lak, a survivor of a destroyed ship. In a standoff Ezri wounds him and, rejecting Joran's urging, calls for medical help rather than killing him.