There are some stories that are better the second time around. A really elegant twist doesn't just reveal something that the audience didn't know before; it illuminates the story in a whole new light, changing the way that you perceive events that you already saw and making you rethink everything you knew. A very good twist makes you want to watch the story all over again to see the story from a whole different point of view.
"The Powerful Enemy" has a great twist, and I'm sorry if you don't know it, because I'm about to spoil it. Bennett is Koquillion. If you don't know that, this is a reasonably interesting episode; the Doctor and his companions encounter a shipwrecked crew trapped by hostile aliens, and have to find some way to outwit them and free the humans. It's the sort of thing that you could expect in this point in the series, and anyone watching it who knew that Maureen O'Brien was going to be the new companion could imagine Bennett sacrificing himself to destroy Koquillion and Vicki going off in the TARDIS.
But if you know that Bennett and Koquillion are one and the same, this is an absolutely terrifying psychological thriller. Because every scene between Vicki and Bennett and between Vicki and Koquillion takes on a whole new light when you know that they're one and the same person. Every time Bennett warns Vicki of the terrible threat that merciless and brutal Koquillion poses to them, you understand that he is terrorizing her. Every time he unleashes his inner sadism while wearing the alien mask, you understand that he is tormenting and brutalizing her into a constant state of dependence on him. When you watch it knowing that Bennett is Koquillion, this is a nightmare.
That brings a whole new meaning to the scene where Vicki meets Barbara. Barbara's sympathetic look feels entirely different, Vicki's panicked insistence that "Koquillion always knows, he knows everything" is absolutely heart-breaking, and all you can think of is how much you want that poor girl to be rescued at once. It gets you on her side instantly and effortlessly. Not that Maureen O'Brien couldn't have done that just with her natural charm, but she's allowed to have a real performance here because of the nuances of the script and she shows what she can do right off the bat. This is definitely Vicki's story, and she steals every scene she's in.
All that makes it sound like not much is happening with Ian and the Doctor, but this is where Hartnell really starts to shine. Hell, he incandesces in this episode. From the beginning, where he shows a new warmth and tenderness to his friends that almost-but-not-quite conceals his aching loss over Susan's departure, all the way to his charming banter with Ian as they escape the temple, he is mercurial and adorably pompous and hilariously dotty and brilliantly sharp ("I can hear you in here") and deductive and everything he needs to be to carry his scenes. It's not that he's never been good before; it's that up until now, this has been an ensemble show and now it's a hero and a supporting cast. Hartnell steps right up, and he's brilliant.
And for all that people complain that "Bennett is Koquillion" is obvious in retrospect, Ray Barrett does an amazing job of making Koquillion seem creepy and alien and menacing. Honestly, my one complaint about the episode is that instead of being involved in the cliffhanger somehow, we instead get a bog-standard deathtrap (from supposedly peaceful aliens--exaggerate much, Doctor?) that we all know Ian's going to get out of a few minutes into the next episode. Still, this is a great piece, well above its reputation as a whodunnit with only one suspect.