This is the one about the vampires. It's a leftover from the Hinchcliffe-Holmes era, which was bumped because the BBC was doing a Dracula miniseries at the same time and because Horror of Fang Rock was just too good not to bump something for. But, what with a whole new TARDIS crew, JNT being afraid of Mary Whitehouse, and Christopher H. Bidmead having been given a mandate to turn the show into hard SF, it ended up pretty different than it would have been in season 15, even though most of Bidmead's changes got undone.
The Doctor and friends are still stuck in the green hot pocket universe. Also Adric, who stowed away on the TARDIS at the end of the last episode. So they land on another planet, because they've got fuckall else to do.
There's a medieval mini-kingdom with a mix of Romanian and Hungarian names, ruled by three lords in a spooky castle. So, yeah, vampire story. And the vampire lords want to revive the Great Vampire, who's the oldest and deadliest enemy of the Time Lords, and it's the sacred duty of all Time Lords to hunt him down and kill him above all other priorities, even though Romana's never heard of him.
Some of the people want to rebel against their oppressors. Others are afraid that will just lead to reprisals that make things worse. There's also some political strife between the bourgeoisie and the working classes, and some between the townies and the rural folk, and some family issues.
Adric sneaks off and gets heavily involved while not actually affecting anything, starting with him getting caught stealing food for no reason, and ending with him becoming the Chosen One the vampires are going to use to awaken their king but not actually doing anything in the ritual. Anyway, this should have been the end of Adric, since nobody but K9 knows he's even there, and the Doctor never listens to K9, but alas…
Meanwhile, the Doctor and Romana wander around telling everyone they're scientists and therefore better than the locals, plus not dressed in filthy medieval crap. Someone throws some rubber bats at them, so they decide to go to the castle to get captured by the vampires. The vampires recognize that Time Lords are superior beings and offer to let them in on the vampire fun, but they refuse.
They manage to escape with the help of some locals, but meanwhile, they've discovered Adric is there (see?), and Romana refuses to leave without rescuing him first. Only four weeks later, she'll decide to stay trapped in a hot pocket universe just to avoid having to travel with Adric anymore, but sadly, she hasn't gotten to know him that well yet. The Doctor is a better judge of character - he spits out "Adric!" in disgust and leaves.
The vampires decide Romana will make a perfect sacrifice, while Adric becomes a willing (or maybe hypnotised; it's hard to tell with his acting) participant in the Great Ritual for the Great Awakening of the Great Vampire. Great rescue, Romana. So they tie her up in their sex dungeon.
The Doctor asks K9 to lead the rebels, and everyone decides sure, a tin dog will make the perfect general. As it turns out, he's not so much a general as an entire army, and the rebels sit around doing nothing while he lets fly with the lasers.
But the Great Awakening is already beginning, and a giant hand reaches out from the ground, because the Great Vampire isn't just a vampire, he's also a kaiju. The Doctor pilots the TARDIS into the castle, which is really a spaceship, and launches the scout fighter to fly up into the air and then crash back down, right through the Great Vampire's heart. Because Our Vampires Are Different, these ones are killed by a steel stake through the heart, not a wood one. So all the vampires to crumble into dust, another successful genocide for the Doctor.
The Doctor decides to give the medieval people a bunch of technological data which they can use to become scientists and therefore better like him and Romana, and leaves to bring Adric home. Which, sadly, he fails at.
Tom Baker was so sick that his hair uncurled, so they had to give him a perm.
The vampires, being a hugely important part of Doctor Who mythology, and connected up to all kinds of Olde Tyme Gallifrey stuff and the Doctor's hermit tutor and so on, never got mentioned again on the TV show, but ended up all over the other media.
People used to call this the most homoyay story in Doctor Who, what with Camilla obviously lusting after Romana, licking her blood, and tying her up, while Aukon is distracted by Adric and whispering "come" in his ear. After Survival, and then the VNAs, and then RTD, not to mention all the Anne Rice and Rice-inspired vampire movies, it seems a lot less blatant. Especially since all the gay subtext is drowned out by Tom and Lalla being unable to keep their hands off each other.
When people talk about "trad" as a good thing, you can't get more trad than Terrance Dicks enthusiastically romping through the darkest background he can come up with, with the Doctor helping villagers overthrow evil overlords that are also a Hammer Horror staple. It's a fun adventure, which successfully merges the horror, silliness, and sci-fi of the three eras of Tom Baker.
But that's all it is. None of the more interesting sciency ideas that make the rest of season 18 work when it works (and fail when it fails). The plot's kind of weak, especially since most of it turns out to be irrelevant in the end. Tom Baker often looks like he's about to keel over and die. Making vampires scarier by making them gigantic is pretty stupid. Dicks did a great job replacing Leela with Romana, and he made K9 useful again, but he had a lot less success finding something to do with Adric. The attempt to shoehorn this into the E-Space trilogy adds nothing to the story, and makes the whole E-Space thing kind of pointless. Also, the sets and effects are a real step down from the rest of the season.
On the third hand, the acting's pretty good. The vampires don't go over the edge into camp panto villainy, and are all distinct characters. The villagers are mostly interesting people that you can relate to. Romana is the Romaniest. ("Do you know that you are wonderful?" "Well, I suppose I am.") And, when he's not dying, Tom Baker is pretty great, hitting the silly bits but also playing serious better than he had in years. And this is even the story where Adric is at his best. Well, least bad. Except for not dying.
Also, Paddy Kingsland shows what a synth and some Radiophonics can do when he's not given a rush assignment at the last second as in most of the next season.
So, sure, maybe it's all been done before on Who, and better - but season 15 didn't have Sexy Romana to tie to an altar.