Fans of Star Trek: Enterprise know that the fourth season has been a milestone in connecting the history of the prequel series with the rest of the Star Trek universe.

We've seen great story arcs including one dealing with Vulcan socio-history, set on their rather inhospitable homeworld, and one centered on Dr. Arik Soong, an ancestor of Data's creator, Noonien Soong. Other episodes include "Daedalus" about the "father" of the Transporter, Emory Erickson, and an upcoming storyline addressing the Klingon forehead mystery. Those, along with a trilogy starting tomorrow night fleshing out Original Series-era aliens — Andorians, Orions, and Tellarites — and presaging the Romulan Wars, give us a mix of stories that demand attention by the long-time fan.

And if these stories aren't enticing enough, perhaps the coup de grâce of Season 4 will be the two-part story "In a Mirror, Darkly," a script that revisits the mirror universe but focuses on the power struggles of Captain Forrest and Commander Archer and the discovery of a rather special starship. This story is the brainchild of writer Mike Sussman, who wrote both teleplays, along with executive producer Manny Coto, who wrote the story for "Part II." According to Sussman, the episode is "a prequel to 'Mirror, Mirror' and a sequel to 'The Tholian Web,' two of my favorite episodes."

The history of getting this episode made included a long gestation period. "When I initially pitched this idea two or three years ago, it was too big for a one-part episode. But Manny suggested we do in two parts, and that was the key to realizing it," says Sussman.

Fans of Vaughn Armstrong will be glad to hear that he reprises his role as Forrest, albeit with a twist. This Forrest is everything that the good admiral from our universe is not. For the most part. The beauty of mirror universe characters, of course, are the subtle nods to their real-universe characters. Archer, as second in command, is given carte blanche to plot his way into command of the ship (remember "Mirror, Mirror") not to mention regaining the woman (Hoshi) who's loyalty changes, depending on whomever is in command.

Playing the alterna-Forrest has been great fun for Armstrong, who's now completed filming his scenes. On his last day, the always jovial actor commented about his "action" as the commander of the mirror starship. "In 25 years [of acting], I haven't gotten the girl, but I come back here and I get the girl! This is great."

Scott Bakula was also able to find humor in Vaughn's return. "Didn't we kill you already?" he joked outside the Enterprise sets.

"Yes, but this death is much better!" replied Vaughn on his mirror character's imminent doom. His "real" character, the upstanding Admiral Forrest, met an untimely but heroic fate saving Soval from a bomb blast at the United Earth Embassy in "The Forge." For a character we had followed for over three years, it was all over very quickly, perhaps too quickly. But the beauty of the mirror universe is that there are second acts, even if it is a dangerous place with a short life expectancy. Vaughn Armstrong, one of the most prolific guest stars dating back to the first season of Star Trek: The Next Generation, may be done for now but he will no doubt be back again one day. For now, we will wait and see what's in store for Captain Forrest and his command in this "Mirror."

On Tuesday, even though it was the last day of shooting for the James Conway-directed part one of "In a Mirror, Darkly," it was historic as well. The highlight came when filming moved on to the Bridge of the Constitution-class Defiant, the one from the TOS era as seen in the episode "The Tholian Web." We won't tell you how the two ships meet, but it is in Tholian space. That's all we're saying for now.

For these historic scenes, production designer Herman Zimmerman was responsible for what is perhaps the most awe-inspiring set done for Enterprise to date. It may not be the most complex set, but the fact that it was done, and done to perfection, is due to a tremendous production team who made sure that every last detail was in place prior to filming. Although it is the Defiant, it is, for all intents and purposes, the Original Series U.S.S. Enterprise. There are subtle differences, but viewers will be amazed at the detail used for this full three-quarters around set, the likes of which have not been used for filming since the final Star Trek episode, "Turnabout Intruder," in 1969.

Special kudos for the Defiant set must go to members of Zimmerman's team, such as set designer Ahna Packard, who spent days drawing the set in incredible detail and also to senior illustrator Doug Drexler, who pioneered most of the original research for the DS9 "Trials and Tribble-ations" episode and the set built for the London Hyde Park exhibit. Truly a group effort, many more details of the Defiant set were worked on by such art department veterans as scenic art supervisor/technical advisor Mike Okuda and scenic artist James Van Over.

Acknowledgement must also go to construction coordinator Tom Arp and his department. After all, they are the ones who physically build the amazing sets you see week in and week out. For the Defiant, they've gone that extra parsec and produced a stunning vision of this older, but at the same time futuristic, ship.

Writer Mike Sussman is impressed by the efforts of the crew in constructing sets for "In a Mirror, Darkly": "I think the bridge set is remarkable. I hope fans will be thrilled to see that set again in all of its glory. I feel that it probably looks better than the original in many respects, if you compare them side by side."

The "In a Mirror, Darkly" saga is set to air on April 15 and 22, on UPN.