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My Sister, Benedict Arnold is the fifth episode of Season 3 of The Brady Bunch, and the 54th overall episode of the series. Written by Elroy Schwartz and directed by Hal Cooper, it first aired 15 October 1971 on ABC.

Short Summary[]

Greg and Marcia use their dates to get back at each other, and the younger Brady siblings prepare for the school carnival by building a dunking tank.


Marcia is excited that a high school boy has asked her out on a date. Marcia tells Greg about the date and wants his opinion about the boy. Without yet telling him who the boy is, Greg tells her that the only boy who he would object to is his school rival, Warren Mullaney. Greg detests Warren, who not only beat him to the last spot on the first-string basketball team by flattering the coach, but also beat Greg in the election to class President with a bunch of empty promises. Marcia's date happens to be with Warren, but based on Greg's comments, she figures that she will still go out with him since it is solely Greg who has a gripe with Warren, but she plans to not tell Greg. Greg finds out that Warren is Marcia's date, and he forbids Marcia from ever going on a date with him again.

Although Marcia was initially not going to continue dating Warren because there was no "spark" between them, she decides to ask him out again solely to spite Greg. However, Greg brings home Marcia's rival, Kathy Lawrence, who beat Marcia to the top spot on the cheerleading squad. Marcia and Greg clash over this, until Mike intervenes and admonishes them both for using their dating partners just to spite each other. Greg and Marcia go to the other room to apologize, but return to find that Warren and Kathy have taken a fancy to each other and left together.

Meanwhile, the other kids are all excited by the upcoming junior high carnival. Alice, Peter, Cindy, and Bobby work on a dunk tank. Cindy and Bobby take turns getting dunked, and Alice unintentionally gets herself dunked as well while collecting laundry she had hung on the tank to dry.