TARDIS Crew: Seventh Doctor (he of the perpetually trilling R's, Sylvester McCoy) and Ace (Sophie Aldred).
The Plot: A distress signal brings the Doctor and Ace to Lake Vortigern in England, where he encounters a UNIT nuclear missile convoy and a whole slew of extradimensional knights who insist that he's Merlin. He isn't - yet - but that doesn't mean that they want to kill him any less. And since UNIT is involved, Brigadier Lethbridge-Stewart (Nicholas Courtney) all but leaps out of retirement to team up with the Doctor one last time, and if he's not careful, it really will be the last time, since the knights' queen, Morgaine (Jean Marsh) - as in the sister of King Arthur - wants to defeat Merlin and her brother (who might just be resurrected) at any cost, whether it involves using her minions, a monster called the Destroyer, or that big nuclear missile currently parked by the lake.
The Thoughts: This serial has its problems - most of them budgetary, with a few random go-nowhere plot elements dropped in here and there - but it's very easy to overlook them because overall, this is a very fun story. For one thing, I'm pretty sure this is the first time in the history of the series that the Doctor has to deal with the fall-out from an adventure he hasn't even had yet (first time in an official TV adventure, anyway). For a show about travel through time and space, I'm surprised it took them 26 seasons to get around to that idea.
Also, it's Doctor Who mixed with Arthurian stuff. Sci-fi Arthurian stuff where someone is as likely to pull out a laser pistol as a sword, but still, that's just cool. I'm not made of stone here.
And it doesn't hurt that it's basically Old Home Week for the show. You've got UNIT, Bessie the car, references to any number of Jon Pertwee and Tom Baker characters and monsters (the Doctor to Ace, when asking her to pose as Liz Shaw: "Think like a physicist."), and of course, the returns of both Courtney and Marsh, notable not only because they've both appeared on the program in the past, but because they played brother and sister Bret Vyon and Sara Kingdom WAAAAAY back in the William Hartnell story "The Daleks' Master Plan," so it's a reunion within a reunion. Neat trick, that.
To say Nicholas Courtney is great as the Brig is as much a foregone conclusion as to say that grass is that stuff that grows out of the ground that I need to mow sporadically. But there's an extra layer to his performance here as the old soldier going back into battle to help his friend not because he's bored and desperate for action (he seems to quite enjoy retirement on his palatial spread), but because that's what an honorable soldier does (that's not to say he doesn't like being back in the thick of things, however).
The idea of martial honor plays a big part in Jean Marsh's Morgaine, too. She loves her some warfare, but she insists things are done properly. When she discovers her son has desecrated a military cemetery, she halts her plans in order to hold a ceremony honoring the soldiers buried there, and invites the Brigadier to join her, no less. They're enemies, sure, and pretty much pledge to kill each other when they next meet, but there's a mutual respect that can't be denied. Morgaine definitely sees a kindred spirit in the Brig, because he plays by the rules as she sees them, whereas she'd like to squash the Doctor like a bug because he keeps trying to change the game (even if, as she claims, she's actually better at the game than him). Morgaine would have been such an easy role to chew scenery with, but Marsh plays it quieter and more restrained, and is all the more sinister as a result. Script and actor combine to make a hell of a villain, which is all the more impressive in a series that's not known for subtlety.
Overall: If you can get past the McCoy-era budget (you'd think UNIT could afford a better helicopter, huh?), a few plot points dropped as quickly as they're introduced (why does everyone lose interest in Excalibur's scabbard?), and some cheesy performances (esp. UNIT's new Brigadier, Winifred Bambera, as played by the woman who was the alternate universe female Dave Lister on that one Red Dwarf episode), you'll probably enjoy this a lot, especially if you're a fan of Jon Pertwee and Tom Baker stories. And McCoy has a few funny moments, nor is he as prone to random shouting as he'll get at the end of the season yet, so that's nice.
A quick word about Ace: Ace is super-protective of the Doctor in this story, to the point that she resents his relationship with the Brigadier and seemingly the very idea that there were companions before her. It's less annoying on Ace than it ever was on Rose, though. For one thing, Ace is looking after someone who is not merely a mentor or father figure to her, but the one authority figure in her life who ever made her feel worthwhile, so her "guard dog" attitude is really sweet. Whereas Rose was just plain jealousy personified, because OMG she luvs the Doctor and he's totally gonna be her Edward Cullen and they're gonna go and make sparkly babies or some shit.
Also, it doesn't hurt that Sophie Aldred is cute as hell.