The Armageddon Factor is the first ever season-arc-finale on Doctor Who, tying up The Key to Time-Wasting in a typical Moffat resolution, spending more time on cool scenes and funny in-jokes and complicated timey-wimey than on making sense, and then throwing in all the big revelations in a rush at the end in hopes you won't realize that they don't so much answer all your questions as make you feel stupid for expecting an answer. Then again, Moffat was only 17 or 18 at the time, and wasn't even the writer, so you have to give him a little slack.