Part 1 here
After two years of episodic stories, in season 3 Enterprise becomes a serial with every week following on directly from the end of the previous episode. As this is more in keeping with the style of modern television shows, this change works well and the episodes provide consistent entertainment. The longest continual story is the Xindi arc, which starts in the final episode of season 2 and is effectively concluded with episode 3 of season 4. This means it's a 28 part story and as such it's the most ambitious of all Trek's stories, even more so than the Dominion arc in Deep Space 9 that was never allowed to dominate to the exclusion of all else. As a story arc it's a success, but with qualifications as several elements could have been so much better.
The strength of the arc is the plot structure, which gives the impression of being well-planned in advance. The aim of the story is a clear one. The Xindi attack earth and kill lots of people, and they'll be back soon with an even bigger weapon that'll kill everyone. Only Archer and his motley band of redshirts can stop the Xindi, except the moment the crew set out on their mission, they discover that things aren’t what they first seemed. Thereafter, the problems keep on mounting. Every week develops on what came before while revealing a new part of the story, and even the interludes with the various aliens of the week always add another vital piece to the overall plot. Best of all the tension and the pace of plot development increases as the situation grows ever more desperate leading to an enthralling adventure.
What stops the arc being as strong as the Dominion War is the Xindi themselves, as they are poor villains. It's obvious the intention was to recreate the dynamics in the Dominion that had Changelings, Cardassians, Vorta and Jem'Hadar, all of whom had different agendas. As Deep Space 9 only needed to get these alien races together to create strong drama, the Xindi are given the same sort of divergent aims. They have five distinct races. On one side there's the nasty Reptiles along with the enigmatic but probably nasty Insects. On the other side there's the good-natured Apes along with the enigmatic but probably good-natured Fish. In the middle, surprise surprise, is the Humanoids who could go either to the dark side or the light. As a dramatic set up this is promising, even if Big Fish In Space does take some believing, but after creating the situation nothing is done with it.
For the first third of the arc the only thing the Xindi do is sit in an interminable meeting and agree they need to destroy earth, now. For the second third, they potter around while generally agreeing they need to destroy earth, now. And it's not until the third act that the Xindi start getting names and with it character traits that let them become people who are interesting to watch rather than mouthpieces that explain the plot.
If the Xindi had been interesting foes, the arc would have been breathtaking, but as it is, the drama all comes from within by detailing the Enterprise crew falling apart as one by one they abandon the abiding principles all Trek crews have followed. Much is made of Sisko breaking the rules in Deep Space 9's By the Pale Moonlight, but Archer steps past that point in episode 2 and everything keeps on getting darker from there.
The strongest run is the build up to and then the resolution after the battle of Azati Prime. I'm sure this mini-arc within the main arc reuses the year of hell concept originally pitched for Voyager. The idea back then was that Voyager's fourth season would have had half the crew being killed and the ship being left derelict in space forcing the survivors to descend into savagery to survive. The studio thought this sounded too interesting for Voyager and so instead they opted for making the year of hell a time travel story with an in-built reset button. Enterprise rekindles this idea and thankfully it doesn’t press the reset button resulting in the next ten episodes all being played out against a backdrop of debris and repair crews and grimy faces.
The momentum after Azati Prime doesn’t let up and with the Xindi finally being given characters with motivations the closing stages of season 3 is probably Enterprise's finest period. Even then though there are niggles, not least of which is the introduction of a species called The Guardians. They are a mysterious race who dress like the Borg queen, who have prosthetics that make them look like the Changeling race, who live in a realm that looks like the null-space Sisko ended up in, and worst of all everyone talks like the Prophets. Even in the midst of Enterprise's strongest drama, The Guardians made me realize that Trek did need a break as clearly all the original ideas for alien races had been used up.
Highlight: The immediate aftermath of the Battle of Azati Prime which does something that no Trek series has done before and shows sympathy for the redshirts who blindly got themselves killed so that no main cast member need die.
Lowlight: Sadly, it's the filler western episode. Much as I wanted to enjoy six-shooters taking on phasers, the story is the only one with no link to the main arc and the story itself is weak. Heck, I've used every element of its story at least five times so I know it's not very original.
Season 3 ends with what is officially the show's goofiest moment. I'm sure it's never been confirmed, but it's safe to assume that with viewers failing to warm to the Xindi and with the show seemingly as doomed as the Enterprise looked at Azati Prime, the makers decided to end Enterprise with a silly cliffhanger just to annoy everyone. On the other hand, Nazi aliens have a rich Trek history so perhaps it wasn't as barmy as it looks.
Season 4 is the final season and it features the last and most successful change of focus. Enterprise's great unanswerable question is what might have happened if the show had been made from the start in season 4's style. Personally I think Enterprise failed to attract viewers because we were all jaded by too much Trek, so it wouldn't have changed anything, but the most striking thing about season 4 is it doesn't look like a show in its death throws. Even though Enterprise was only reprieved for a year to get the episode count up to 100 so it could go into syndication, the lack of faith by the studio and the viewers alike doesn’t show on the screen.
Firstly the Xindi arc, temporal cold war arc and Nazi alien nonsense are wrapped up in an appropriately nonsensical manner. Amusingly when the time lines are reset I noted that Maggie Thatcher's rise to power was one of the key elements in creating the world of tomorrow that needed to be restored, but then again so was Hitler's rise to power.
With the goofy element over with, Enterprise does something original for Trek by presenting a linked series of multi-part stories each lasting 2 or 3 episodes, all featuring in-depth exploration of alien races and ideas from the original series. So there's stories about Andorians, Vulcans, Tellarites, Klingons (both the crinkly- headed type and the greasy moustache type), genetically improved humans, with a side order of Organians, Tholians and even the Gorn.
Most interestingly of all several episodes look at earth and how humans react to the new galactic order. These stories are more complex than anything that was ever attempted involving these races in the other series where the need to wrap everything up neatly in an hour usually ensured a superficial approach. There's even a growth in the fleet with the launch of a second starship, the setting up of the groundwork for the formation of the Federation, and new potential arcs such as a trip to the barking mad alternate universe and a welcome look at the birth of Section 31.
Highlights: Brent Spiner displays a lot of charm playing his maker's granddad, even if his story is let down by the genetic superhumans being so inept. Shran becomes a semi-regular and gets to display a wider range than before with humour and a gentler side. And the alternate universe has the crew on a Kirk era starship, which is fun and nostalgic.
Lowlights: The ill-considered homage to the original series featuring Orion slave girls that invokes the vibe of Mudd's Women, which is a vibe nobody wanted recreated, even if it does let Mayweather explain how he got big biceps as a teenager. And of course the final episode. Like most of the failures in Enterprise, I can see what they tried to do and why, but that doesn’t excuse ending the show with a missing episode from the Next Generation featuring a podgy Riker making pies with holodeck versions of the cast before declaring an end to the program. And Trip deserved better.
Bad final episode aside, season 4 is one of the strongest of all Trek seasons, and following on from the consistent ending to season 3, it's clear that the show was axed in its prime and that it would have only got better. To be fair, any comparison of the show to other spin-offs should therefore only judge it against the other spin-offs' early years: Next Generation before the Borg arrived, Deep Space 9 before the Dominion War, Voyager before Seven arrived. And when that's done, Enterprise fairs well.
Yes, the criticisms usually hurled at the show are valid: the theme song is jarring, the temporal cold war is nonsense, season 2 drifts aimlessly, the Xindi are awful, some continuity errors such as using the Ferengi are ill-judged, and the ending should be condemned. But that's not the whole story. The original series wasn't all like Spock's Brain, Wesley Crusher didn't save Picard's butt every week, while Voyager shouldn't be judged on Fair Haven alone. Enterprise was the same. It wrapped up the temporal war, season 3 and 4 have purpose and focus, and when it got the continuity right, such as the mirror universe being a sequel, prequel and reboot all at once it was very good indeed.
It wasn't to be though, and annoyingly season 5 could have been great. It was planned for the multi-part arcs to grow into a large arc about the formation of the Federation. Shran would have joined the cast as a regular and other ideas planned involved the birth of the Borg queen and a trip to Larry Niven's Ringworld, and if that wouldn't have excited science fiction fans, I don't know what would.
As it is, Enterprise is one of the shorter Trek series, but it's unfair to view it as having killed off the franchise. The franchise was ready to end due to over-exposure and Enterprise happened to be on the screen when apathy finally set in. Its axing didn’t occur due to any failings in the show itself. If anything, its merits hint at what the franchise can achieve if it's ever revived for tv.