|(via Memory Alpha)|
Before we start, though, I'm going to provide a bit of an info-dump for the benefit of anybody who has never seen the latter show. Deep Space Nine ran from 1993 until 1999 and differed from all of the other Star Treks in that it was set on a space station situated near an important wormhole. In a nutshell, the latter seasons of The Next Generation (DS9's predecessor series) featured episodes focusing on an alien empire called the Cardassian Union. There was quite a bit of friction between the Federation and the Union do in part to the Cardassian's occupation of the homeworld of another alien species called the Bajorans. It's stated that 50 years prior to the start of DS9 that the Union had conquered the Bajorans and used them as slave labor to strip mine their planet (the Cardassians were incredibly resource poor and apparently just had crappy luck finding good planets in their part of space). The Union eventually agreed to leave the planet under pressure from the Federation, which of course rushed in to provide aid to the Bajorans as they tried to piece their civilization back together. The Cardassians left behind a space station called Terok Nor and the Bajorans, not really being capable of maintaining a station, handed it over to the Feddies until they could. Starfleet renamed it Deep Space Nine and crewed it with whoever they could find that didn't much care about being sent to what was essentially a backwater posting.
|"Hasta la vista, baby!"|
(via Memory Alpha)
Yadda, yadda, yadda, then the Dominion War broke out, pitting the Federation, Klingons, and Romulans on one side, and the Dominion and Cardassians on the other. I won't go into detail on that because we're getting off track, but yeah, the good guys won.
Anyways, back to "Field of Fire". The premise of the episode is that someone is going around DS9 and murdering Starfleet officers. The catch: each of the three victims were killed in their quarters with a ballistic weapon at long range. It was initially thought that they were killed at close range until Odo, the constable of the station and an aficionado of 20th century crime novels points out that the first victim didn't have any powder burns consistent with a close range shot. It becomes a locked room mystery because there's no evidence that anyone else was in the quarters were the victim at the time of the murders. The psychological thriller element comes into play when one of the characters, Ezri Dax calls upon the memories of a prior life.
Hold on, let me explain. Ezri is member of an alien species called the Trill. A very small percentage of Trill have the ability to "bond" with a weird slug-like species called Symbionts. When the latter is surgically implanted into a host Trill's body, their minds and personalities blend. Ezri is still Ezri, she just has the memories and experiences of every previous host of the Dax symbiont. Every time a host died, Dax would be transferred to a new host. It ended up in Ezri because its previous host, Jadzia, was killed in a very violent manner and it almost died while being rushed back home to Trill. Ezri being the only compatible Trill (or possibly the only Trill) on the ship at the time, became the new host. It's actually one of the coolest things in Star Trek.
That prior life that Ezri calls upon is that of Jaron Dax, the one personality that all subsequent hosts had suppressed because Jaron was a murderer, having taken three lives before he was caught and the symbiont was removed.
I'm getting a bit ahead of myself here. Let's go back to the murders and examine the weapon used. It's quickly determined that the murder weapon is a TR-116, a prototype rifle developed to operate in environments that phasers couldn't due to shielding or radiation. Starfleet dropped the weapon before it went into production after somebody figured out how to make phasers work in those conditions. But, for reasons unexplained, the rifle was included in the database of things that a replicator can create and that's the route the killer went. Oh, and it turns out that the only people who can access the plans for the rifle in the database are Starfleet officers.
|(via Memory Alpha)|
Well, with the "how" out of the way, Ezri has to determine the next two important questions: who and why? After the second murder, she performs a ritual to bring Joran Dax's memories and persona back from its hidden depths to help her gain perspective on the act and art of murder. As I said, this is the psychological thriller aspect of the episode and its pretty good. Throughout the episode, Joran constantly tries to tempt and persuade Ezri to put herself in the role of a cold-blooded killer in order to catch the one she's after...and he almost succeeds twice, which would have resulted in her taking an innocent life in the first incident. Not surprisingly, since she was one of the main characters of the show, she resists and does "get her man".
Who was the killer? A Vulcan! Yes, someone from the same alien race as Spock was committing the murders. Ezri and Joran determined that the killer was likely a doctor and scientist. They also found a single connection between all three victims: they each had a picture in their quarters of them laughing with friends or family. This leads Ezri and Joran to suspect that the killer has suffered some kind of emotional trauma and those pictures are somehow triggering a violent response. Just to cut to the chase a little, they discover that a Vulcan science officer named Chu'lak had previously served aboard a starship that recently been destroyed during the war, with him and only five others being the survivors out of a crew of 1250. Ouch, that's bound to screw even a Vulcan up. Indeed, when Ezri asks why he committed the murders, his response is "because logic demanded it." Wow. For a Vulcan to twist logic - something that they prize dearly - into something that "justified" their acts of murder shows how traumatic the Dominion War was.
From there, Ezri dons the scanner, picks up the rifle and checks up on Chu'lak, only to discover that he's similarly equipped and taking aim! Fortunately, she fires first and wings him. When she arrives at his quarters, she has to again resist the urge to kill that Joran is trying to push her towards, but she resists and at the end of the episode, performs a ritual to rebury Joran's persona, but with the caveat that she can't suppress it as deeply as it had been before and will have to live with the effects he's left on her psyche.
I've only ever watched this episode once, but it was always one of my favorites because it was so different from the rest of the pack. Ezri was much more inexperienced than Dax's previous hosts and was still, I think, adjusting to being joined. Would Jadzia have called up Joran to help solve the case? Probably not, but that's what makes this episode so good. Ezri went down a different and dark path and nearly succumbed to it.