If there's one thing that Terry Nation has a gift for, it's pointless brutality. Um...I mean that in a good way...
It's true, though. One of his greatest skills is in showing unflinchingly a convincing and realistic portrayal of the nasty side of adventure, and "Day of Reckoning" spares no sensibilities in exploring the consequences of Dortmun's defiant stand against the Dalek conquerors. The episode opens with the raid itself--chaotic and violent, filled with death and desperate flight from an enemy that has proven to be invincible. (The sound effects of Dortmun's bombs, which sound like nothing so much as glass Christmas ornaments when they hit the ground, are perhaps not quite right for this, but I'm not sure how exactly you do foley a dud bomb not exploding when it hits the ground.)
The raid also makes the usual splitting up of the cast feel a little less like authorial fiat and more like the entirely believable result of an utter rout. Ian winds up trapped on the Dalek saucer heading for the mines, the Doctor is dragged off semi-conscious by Baker, Susan (whose ankle appears to have healed up completely between episodes) hides out in the rubble with David, and Barbara returns to the rebel hideout and watches the fallout of the utter failure sink in.
Well, sink in for some. Dortmun's optimism now borders on the delusional--he's convinced that all he needs is another few tweaks to his formula, another few volunteers to toss the bombs (and does it not sink in to anyone that he's claiming his bombs can shatter the otherwise-impervious Dalek armor, but that they can toss them from about ten feet away with no ill effects?) and they'll have their long-awaited victory. Tyler, who has already capitulated to Dortmun once and sent 90% of the previous strike force to their horrible deaths, has pretty much had enough and fucks right off out of the story for a while. Really, Jenny and Tyler's reactions combine to really bring home the reality of the situation--they're both equally cynical about their remaining chances, but they express it in different ways. Tyler abandons his former comrades, deciding that caring about anyone but himself is a waste of time, while Jenny pretends not to care about anyone while showing through her actions that she still cares all too much. The expression on her face at Dortmun's death speaks volumes.
Actually, it would have been fascinating to see Jenny replace Susan as a companion, rather than Vicki. The interactions between her and Barbara, based on a sort of frustrated inability to appreciate each other's point of view, are really some of the best scenes in the whole episode. The best scene, though, has to be Dortmun's death. It's really the culmination of everything Nation's been trying to achieve in the story--his last stand is defiant, heroic, a demonstration of the indomitable human spirit, and utterly useless. The shot of the grenade, detonating with a futile hiss as the Daleks stand confused in front of his broken, lifeless body, is probably the best thing Richard Martin ever did.
Oh yes, and Hartnell's still in the series. He does a better job in this episode, and so does Nation; the scene where he and Susan argue is a bit clunky and clearly setting up her departure in a few episodes' time, but the subsequent bit where David flatters him shamelessly and the Doctor pretends that listening to David's advice is his own idea is the kind of charming egotism you expect from the Doctor, rather than the tiresome arrogance we saw in the previous episode. There's even a nice moment between Baker and Campbell that convincingly sells a friendship between the two, right before the former's departure...and murder, two seconds later, by a Dalek patrol. Did I mention the pointless brutality? Even Ian gets a taste, as he kills the Roboman who was his cellmate an episode ago (although that scene would probably have worked better if they'd mentioned that in the dialogue). Ultimately, Nation succeeds brilliantly at making this story look like a genuine war, occupation and rebellion. It's no wonder he went on to to explore the same themes elsewhere--being grim is kind of his skillset.