''You have to enter a specific kind of mental space when you watch the show,'' says Christopher Knight (a.k.a. Peter) on The Brady Bunch: The Complete First Season. True, true. Even folks who tuned in during Brady's original run (how I've envied them all these years!) must have found the sugary series strange to swallow at a time when X-rated movies were winning Oscars and Vietnam dominated the evening news. No wonder the Shakespearean-reared Robert Reed suffered so! But on second well, more like 102nd viewing, he's the one who shines here, in the show's purest, most high-jinks-free installments (Tiger runs away, Marcia gets braces, Greg's hot for teacher, etc.). EXTRAS With so much already out there, it's hard to complain about a dearth of bonus features. Still, the transfer is so crisp that you can see creases in the backyard AstroTurf. Understates Barry Williams (Greg) in one of his two fun, self-mocking commentaries with Knight and Susan Olsen (Cindy): ''I can't behave our show never won an Emmy!'' One of history's great injustices.