"Fright Night" is another one of my favorite Halloween episodes. The Brady boys and girls take turns scaring each other with Halloween pranks, and after the parents are awakened in the middle of the night for the second time, they make the kids promise not to scare each other anymore. Taking advantage of the oath's loophole, the kids conspire to scare Alice, who has made it known that she isn't scared by characters like Dracula and the Wolfman. The kids plan to surprise her one night when Alice returns home, expecting her to arrive well before Mr. and Mrs. Brady come back from a sculpture competition. Needless to say, things don't go quite as planned, and poor Mr. Brady's head ends up in pieces. This episode from Season 4 originally aired 27 October 1972.
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Other Brady Bunch episodes that involve the kids planning to scare someone are "To Move or Not To Move" and "The Slumber Caper," both from 1970.
See a scene-by-scene breakdown of these three episodes at The Haunted Closet