While Star Trek: Enterprise could have been able to sustain several more seasons, United Paramount Network (UPN) canceled Enterprise after season 4. Although ratings for Enterprise were strong in season 1, they dipped in season 2, and creative changes in season 3 were not enough to raise ratings. For Star Trek: Enterprise season 4, Manny Coto was brought in to replace Brannon Braga as showrunner, but, ultimately, it wasn't enough. Nevertheless, there is a lot to love in the existing four seasons of Enterprise, along with some details that can justify the lower ratings.
4 Star Trek: Enterprise Season 3
24 Episodes, September 10, 2003 - May 26, 2004
Season 3 is the worst season of Star Trek: Enterprise. After ratings fell in season 2, Enterprise season 3 was reworked to focus on a more serialized narrative and the existential threat of the Xindi in order to raise ratings and broaden appeal; but it didn't quite work. Enterprise season 2 ended on a cliffhanger after a Xindi superweapon killed seven million people on Earth, and season 3 was dedicated to the search for the Xindi primary weapon. So, Star Trek: Enterprise season 3 was dominated by a single-story arc.
Star Trek: Enterprise season 3 took on a much darker tone than previous seasons. While shows like Star Trek: Deep Space Nine and Star Trek: Voyager were no strangers to showing the darker side of Star Trek, Enterprise season 3 hit differently. For one, DS9 and Voyager still had other, lighter, episodes to break up the overall tone of the darkest seasons of the shows. Since the Xindi weapon hunt was the only arc in Enterprise season 3 meant that, if viewers grew bored of the Xindi storyline there was nothing else interesting to tune in for.
Captain Archer never faces real consequences for actions that would be war crimes in other Star Trek shows.
But perhaps the most important reason Enterprise season 3 fell flat was the way the narrative treated Captain Archer. Because Enterprise is set before the founding of the Federation, Captain Archer and the Enterprise were operating with very little oversight. In the episode "Damage," for example, Captain Archer steals a warp core, stranding another ship in a dangerous region of space. Episodes like that are compelling on their own, but the fact that Enterprise's Captain Jonathan Archer never faces real consequences for actions that would be war crimes in other Star Trek shows makes season 3 disappointing as a whole.
3 Star Trek: Enterprise Season 1
26 Episodes, September 26, 2001 - May 22, 2002
Enterprise season 1 has the highest rating on Rotten Tomatoes of any season of Star Trek: Enterprise, and the overall quality of Enterprise's inaugural season lives up to its reputation. Some of the arcs that work best in Enterprise season 1 are the conflict between the Andorians and the Vulcans, and the corresponding uneasy alliance between humans and Vulcans. Since all three species are founding members of the Federation, their being so divided at the start of Enterprise created instant intrigue.
Perhaps the biggest drawback to Enterprise season 1 is the Temporal Cold War, which continues throughout the rest of the series. 5 of the 26 episodes in Enterprise season 1 are devoted to the Temporal Cold War - more than any other season. The Temporal Cold War was an invention of Enterprise, and in some ways, it felt like a distraction. The most interesting plots in Enterprise season 1 involve establishing humanity in the galaxy, and the Temporal Cold War pulls attention away from those stories.
Nevertheless, the high points of Star Trek: Enterprise season 1 are iconic. Perhaps Enterprise season 1's best episode is "Dear Doctor," where Captain Archer and Doctor Phlox (John Billingsley) grapple with how much help to offer a pre-warp civilization. In the end, Captain Archer has one of the most iconic lines in all of Enterprise, setting up the Prime Directive for the rest of Star Trek.
Someday my people are going to come up with some sort of a doctrine: something that tells us what we can and can't do out here – should and shouldn't do. But until somebody tells me that they've drafted that directive, I'm going to have to remind myself every day that we didn't come out here to play God.
For that line alone, Enterprise season 1 earns its place in Star Trek canon.
2 Star Trek: Enterprise Season 2
26 Episodes, September 18, 2002 - May 21, 2003
These early tensions between the Klingon Empire and Starfleet are the seeds from which that conflict will bloom.
Enterprise season 2 thankfully moves away from season 1's focus on the Temporal Cold War, and that shift makes room for other, more interesting plotlines that explore the early days of Starfleet. For example, a major conflict in Enterprise season 2 is the openly hostile relationship Captain Archer forms with the Klingon Empire. In Star Trek: Discovery's 23rd century, the Federation will have an open war with the Klingon Empire. Enterprise's early tensions between the Klingon Empire and Starfleet are the seeds from which that conflict will bloom.
1 Star Trek: Enterprise Season 4
22 Episodes, October 8, 2004 - May 13, 2005
Season 4 is not only the best season of Enterprise, but also one of the best seasons in Star Trek. Much like Deep Space Nine and The Next Generation, Star Trek: Enterprise really grew into its own as it progressed, even if Captain Archer didn't grow a beard. Once Manny Coto took over as showrunner, he introduced mini arcs, each of which took a few episodes to examine the most interesting storylines in Enterprise. From the inverted-theme song introducing the Mirror Universe, to the increasing philosophical and political conflicts on Vulcan, season 4 took Enterprise where no one had gone before.
Star Trek: Enterprise season 4 offers the earliest insight into the Mirror Universe and its history.