The script you reference is from the classic 1975 film titled: Monty Python and the Holy Grail. 
Many Monte Python films consisted of loosely connected skits, frequently veiling an underlying theme.

In this case, the Knights Who Say "Ni!", also called the Knights of Ni, is one of the more famous segments of the film.

The reference to a "Shrubbery" has long and storied retelling in many different social circles.
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