In brief: Many familiar elements, but a very solid episode nonetheless.

"Twilight" has a storyline that will be instantly recognized by anyone familiar with the film Memento (a masterpiece of narrative construction that you should rent immediately if you have not seen it), and then proceeds to add the sci-fi angles, taking on a "what if" parallel-timeline premise that can be instantly recognized by anyone familiar with TNG's "Yesterday's Enterprise" (among others) -- except with the stakes becoming nothing less than humanity's existence itself. If I were a cynic I might say that I've already seen elsewhere most of what "Twilight" has to offer.

But "Twilight" repackages the material well, plausibly ties it into the current Enterprise story arc, and ups the action quotient to literally apocalyptic levels. All the while, it conveys an intimate character story that works in its own right. The Little Character Drama merges with the Big Action Spectacle and it all somehow holds together and seems justified and compelling. This is an episode that has something for everyone.

The episode begins with the disturbing -- if perhaps overreaching -- image of Earth being destroyed by the Xindi while the crew of the Enterprise watches helplessly from orbit (a presumably very distant orbit). This is like the opening teaser of "The Expanse" taken to the nth power, with a massive Xindi sphere emerging from nowhere and swiftly obliterating the planet. The potency of this image is almost justification for its presence in the episode ... since, logically speaking, I don't see how it could actually happen this way. If you follow T'Pol's dialog that takes place 12 years later, you might, like me, be at a loss to explain how the Enterprise could've possibly been anywhere near Earth to witness its destruction. (And, furthermore, you might wonder why the Enterprise was not the very next target after Earth.)

But it's a hook that's probably necessary given the weight of the episode's central situation -- the last desperate gasps of humanity trying to survive -- so I suppose dramatic weight should take precedence over the technicalities of plausibility. The episode is told from the point of view of 12 years in the future, where Archer wakes up in a strange place and finds himself 12 years older than he last remembers. He's unable to recall anything after having been hit by an anomaly in the Delphic Expanse.

This anomaly, T'Pol explains, left behind parasites that interfere with his brain's synaptic pathways. Thus, like the central character in Memento, he cannot form any new memories. After a few hours, any memory formed after the accident fades away, even though he retains all memories from before the accident. The notion of being lucid and perfectly cognizant, and yet trapped by the logic of this situation, suspended in a state of life forever interrupted -- it's deeply disturbing to ponder, and hard to imagine how that would actually feel. Perhaps it would be like it is here for Archer, who experiences such a logical disconnect between his last memory and the current time that there's little opportunity for him to dwell on his condition; he's too busy learning that the condition exists and pulling the pieces together.

Despite years of trying, Phlox was never able to remove the parasites, because they exist, like DS9's Prophets, in a different zone that somehow transcends space and/or time. The only known way to destroy them would be with a subspace implosion that would kill Archer in the process. Astute viewers may quickly identify this as the solution to the entire plot, cleverly hidden in plain view.

Archer wakes up to find a very different -- and yet very much the same -- T'Pol making breakfast in his kitchen. As she explains his condition and the highlights (or, more accurately, lowlights) of the past 12 years, we are supplied a flashback narrative that documents the key events following Archer's affliction. Archer was eventually deemed unfit for duty and relieved of command, and Enterprise continued the mission to find the Xindi under T'Pol, who was granted a Starfleet captain's commission. Closing in on the location of The Weapon, Enterprise was increasingly besieged by Xindi attackers.

One particularly nasty attack forced T'Pol to ram the attacking ships, in an act that I find particularly interesting because it smacks of impulsive, un-Vulcan-like desperation, even if there is a logic that can be argued behind it. But don't bother trying to explain that logic to Trip ("What the hell were you thinking?"), who reports that the warp engine damaged in the crash will take him six months to fix. With the ship crippled, this makes it impossible to find the Xindi weapon before it is deployed.

(Digression: Why was Travis not piloting the ship, you ask -- or perhaps you don't? Because he was apparently KIA, which I find amusingly pathetic. It's like the writers intentionally steer him out of scripts at every possible turn. In the case of alternate timelines like this one, all they have to do is have him lie dying on the floor early in the proceedings, without needing so much as a line of dialog addressing it. But never mind my tired Travis-is-a-cipher speech, blah, blah, etc.)

Earth is consequently destroyed, as is every human colony the Xindi can hunt down. Less than 6,000 humans remain, and they journey to settle on Ceti Alpha V -- a planet whose ear-dwelling indigenous life, unseen here, makes you wonder whether those 6,000 survivors have a new problem to deal with on their new home.

This is a very bleak scenario, and an interesting one worth watching. Since obviously Earth won't actually be destroyed and the Xindi will at some point have to be stopped, "Twilight" permits us an imaginative look at the story arc's hypothetical worst-case scenario. The flashback structure of the "what if" future sometimes reminded me of DS9's "The Visitor" -- although it must be said that "Twilight" is a substantially less poignant take on hypothetical material. (The loss of a parent evokes emotions we can understand, whereas Earth getting blown up is clearly reaching over the top into fantasy.) You don't quite get a real sense here that Earth's destruction is a cause for the unbearable anguish that it should be, because there's simply too much story to tell to dwell on people dealing with unimaginable despair. (Notably, Soval's matter-of-fact attitude toward humanity being wiped out seems awfully devoid of regret, even for a Vulcan.)

Rather, the emotional/character selling point resides less in humanity's destruction than in the nature of the relationship between Archer and T'Pol, after a decade of her serving as his caretaker. "Our relationship has ... evolved," T'Pol explains. Indeed. After 12 years, you would expect it to, even if Archer doesn't remember one minute of it. The nature of T'Pol's feelings for Archer are never explicitly stated, and it's left ambiguous as to exactly how deep they run. I think that's the right choice. Part of her feelings certainly stem from a sense that she owes it to him, since she was indirectly responsible for him being afflicted by the anomaly. But it's clear that there's more to it, and that she has grown attached; after 12 years, being with Archer has become a normal part of her life. Jolene Blalock and Scott Bakula find the right notes for their parts in this strange routine: T'Pol long accustomed to it, while Archer finds it brand-new every morning.

It can't be easy, and you can sense in Mike Sussman's script the allegory for people who have mental illnesses and the people who care for them (Alzheimer's Disease being the most obvious parallel) -- there's a human toll in maintaining patience, dedication, and making daily sacrifices.

Of course, this being sci-fi, there's ultimately a cure here, and this cure also can change history. Because of the odd space/time properties, eradicating the parasites in the present also turns out to eradicate them in the past, which means an alternate timeline would emerge if the parasites were destroyed, thus having never incapacitated Archer. On this particular day, Phlox is arriving with a possible treatment he's been working on for the past decade. Archer returns to the Enterprise to undergo treatment. But, of course, this being an action episode, Phlox's solution is not carried out before an all-out Xindi assault that spells the certain end of the last remaining human colony, as the Enterprise is pummeled and hammered and boarded and the bridge is blown up and the officers are sucked into space.

The final act crescendos into escalating disaster, like "Yesterday's Enterprise" ramped up to our current decade's action standards. Eventually, all the characters are blown up or shot by Xindi soldiers, and humanity's fate lies in T'Pol and Archer setting off a subspace implosion in engineering while being shot at from all sides. The whole ship goes up in a big fireball, which serves as the biggest explosion able to set timelines right since Voyager's "Year of Hell, Part II."

In story theme and sometimes in method, "Twilight" invokes a long list of its older siblings' classic predecessors: "Tomorrow Is Yesterday" and "The City on the Edge of Forever" from TOS; "Yesterday's Enterprise" and "All Good Things..." from TNG; "The Visitor" from DS9; "Timeless" from Voyager. It betters none of those examples (which comprise some fine company), but it does work as another iteration on the material, and it finds a workable balance between its extreme disaster scenarios and more personal moments. I tend to prefer these shows when they have a witness in the story that remembers at least some of what happened (or could've happened), but that's by no means mandatory. After all, we in the audience are the witnesses that count.

Next week: The good, the bad, and the Enterprise.

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Next episode: North Star