On paper it's an odd pairing. There have been various suggestions that Paul Darrow and Jacqueline Pearce weren't getting along at the time - they appear on screen together properly in just one scene by my reckoning in "Gold"; there are a couple of occasions where they're in the same place but not the same shot (e.g. the slave market in "Assassin"). To be fair, she doesn't actually share many scenes with most of the crew this year - which might have been because of some sort of cast schism or might have just been a conscious decision that she'd been overexposed in Season 3, especially in all the face-to-face stuff with the regulars, which was stretching the credibility of why neither side picked up a gun and just shot the other.
There's even been some suggestions that Avon was originally meant to be in Tarrant's role here but if it was the episode has been rewritten impeccably and basically nothing would work if they were to switch. Also by this point even whatever weird frisson passed between Avon and Servalan in Season 3 seems to have gone; the events of "Terminal" seemed to break him and finally take things too far into personal territory by exposing him as gullible and idealistic, while for her part he's just a failure in a planet hopper now rather than a dangerous adversary in the most powerful ship in the galaxy. She won their particular battle of the sexes, which makes him of no more interest to her than anyone else she's beaten.
Tarrant then is something like unconquered territory for her and she's mentioned him a lot more than she has, say, Vila. Plus there are signs she has a bit of a thing for rugged guys a foot taller than her and might actually like him; their relationship is considerably more wholesome than hers and Avon's ever would, though you can ask how much of it is her playing Tarrant (though she's alone in her final scene - while also seeming to shut the door in her own mind) and how much is the influence of Virn. Her feelings for Don Keller seem genuine enough so she's probably not totally without heart and Tarrant's simple personality would mean she could actually relax around him more than Avon. The actors certainly have a surprisingly plausible chemistry and balance; I've not done the maths but it feels like Pearce's longest appearance of the year and certainly her most integrated, while Steven Pacey is great as well; it's a shame as it shows both still very much had it given the right material. The worst thing about Season 4 is the waste of it all; if the show had just gone shit it wouldn't be so bad but the last 4-5 episodes of the year just shows everyone had got complacent and fed up with poor material.
Servalan has a good episode all round; she has another nasty underling trying to use her in the form of Reeve, who comes across with the right arrogance, ambition and brutality in his small role; his bullying of the hapless pilot and his underling's clear hatred for him (compared to, say, the way Travis' troops seemed to think of him if Parr was anything to go by) set him out as a piece of shit and it's no surprise when he tries to turn the screws on Servalan too. She's unruffled, which adds layers to the nicer stuff with Tarrant as she's clearly capable of remaining confident on a 1-on-1 situation with an armed hostile male rather than just putting out as a tactical move. Her role as head of the pacification programme is an interesting one too; considering what we're given again suggests the drug method has replaced pursuit ships and troopers as the Federation's main method of empire building she effectively has her old job back. Though considering she's again recognised, has she not considered a wig or an alias that doesn't sound like a Terry Nation-esque partial anagram of her real name?
On the other side of the pairing Tarrant spends most of Season 4 thinking with his dick whenever there's a woman on offer (in contrast to Season 3, where he seems utterly uninterested despite his machismo - while he is shown to have some affinity to Dayna as a fellow rookie it's only when it suits him and he seems genuinely perplexed at the idea they'd even simulate sex in "Ultraworld") so it's little surprise he'd fall for the doe-eyed version of Servalan. He seems to genuinely fall for her for certain and it brings out the full Tarrant gamut as he's charming, manly, macho, impulsive (getting Dayna to teleport back up simply because he fancies hogging any glory to be found for himself) and thick enough to let her get the gun. At the same time he figures out what is going on with the sand on Virn; he's never really been stupid-stupid, there's just this complete inability to foresee the consequences of his actions. Also 110% Tarrant is him inexplicably telling the crew when back on the Scorpio that the woman on Virn with him was Servalan for absolutely no gain, when Avon and Dayna hate the her, Vila's no fan and Soolin has next to no idea who she even is. Was he expecting a high-five off the girl who's dad Servalan shot?
Considering the rest of the crew spent most of the episode in the atmosphere onboard a disabled Scorpio they don't do badly either (thanks to great direction again from Vivienne Couzens). As in "Headhunter" just dimming the lights makes Scorpio a much more promising setting and the result puts everyone on edge. This is the episode where Vila gets absolutely fucking smashed as opposed to the more functional alcoholism, moping about Cally and snapping at Avon, who treats him with something approaching gentle, weary care. He also gets to crack the mystery parallel to Tarrant on the surface and come up with a decent on-the-fly way out of the whole problem, saving those below as well. Soolin is sound more natural again and it's nice to think the crew have actually talked to her about Cally and other past adventures, while Dayna gets a respectable showing, firstly having a bit of a spine before getting run over by Tarrant's chivalry and then rolling her eyes at drunken Vila and then hungover Vila.
The episode's cheapness works for it as well. Combined with the lack of music beyond the most basic cues on Virn there's a dreamlike quality to it all, which fits with the painted backdrops and dry ice pressed into service. It's a good idea for a plot too, very proper sci-fi, with a decent mystery element. B7 hasn't done a lot of non-humanoid life forms and really explored the wider possibilities of aliens much, so this makes a refreshing difference to somewhere being populated by human looking people in slightly jazzier jumpsuits. The tone is just about right here; it's not quite as unconventional and strange as "Sarcophagus", feeling more like an organic B7 story but here that's a strength. It even manages to be portentous without being pretentious.
Overall then it's a very fine episode, with a strong plausible romance among the chaos between Servalan and Tarrant with a very well done set of consequences and repercussions. The unusual atmosphere works well enough, the crew don't sound like they're reading from a script, it's inventive and it gets in close to most of the characters, even the one-shot Federation lackeys. Even Don Keller gets a personality and he's dead. Smashing stuff more in line with the earlier quality the show gave us, and a welcome revisit to the show's former trademark of not letting a low budget stop it from innovative storytelling and complex characterisation.