Nominated for a Hugo, the most viewed episode of the season, and highly polarizing, "A Night in Sickbay" is an episode that at worst is hard to ignore.
For simplicity's sake, it's known as the Porthos episode. Porthos was Archer's pet beagle, beloved by all, except when featured in his own episode. The Kreetassans, previously seen in the first season episode "Vox Sola, put Archer through the diplomatic wringer when they become offended at his in.advertant insults and force him to jump through hoops in order to make amends. If that's not bad enough, Porthos is another unwitting victim and undertakes the eponymous situation under the care of Phlox.
Of course, it can't be left that easy. Archer spends most of the night there, too, and because even that would be too easy, he spends a great deal of that time obsessing over his relationship with T'Pol. The Vulcan science officer was the subject of a quasi love triangle with the captain and Trip, though things never progressed that smoothly for any of them. The third season saw the most complicated developments, notably in "Twilight," an alternate timeline episode that sees Archer and T'Pol grow old together (and yes, that's a great deal more complicated, too) while the Vulcan and Trip...well, that's not something that ends smoothly.
Anyway, the point is that "Sickbay" is easily the most involved look at the mechanics of the mission probably from the whole series. Fans who like to be ornery will find it ridiculous, but its nuances are what those fans deride. They're exactly the kind who consider Jar Jar Binks to be offensive, which is like saying anything remotely foreign must surely be a joke, which is itself offensive.
Basically "Sickbay" is considered character assassination by these fans. They consider Archer to be a buffoon in the episode. They apparently don't consider a good humbling to be, well, a good thing. It's half of what the Kreetassans have been asking for since their last appearance, and most of what T'Pol and the rest of the Vulcans have been saying since the start of the series, that Starfleet needs to take a more sober approach to its mission. I understand that sometimes it's just more fun to watch lessons being learned while space battles take place, but that's just not going to happen all the time. Enterprise did a surprising number of episodes where such lessons had to be learned the hard way, with a large piece of pie. In that sense, "Sickbay" is no different from "Dear Doctor" from the first season. Watch both of these and you'll understand exactly what I mean.
This is an episode that squarely fits in the "Threshold" tradition, a Voyager episode that a lot of fans consider to be among the worst of the franchise, but is so notable that half the reaction also indicates how hard it is to ignore. That will only happen if there's something worth watching. And much of later Star Trek is governed by fans who hated what they watched, but watched anyway. Yes, that's the "Spock's Brain" effect. As early as the last season of the original season, fans were started to decide standout episodes were not universally a good thing. How odd it is...
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Memory Alpha summary.