By the time you use "THE plot," your both already talking about a specific movie.

it could be the plot of the movie, if the cartoon, of the play

of the*

If you want to be good in English, you have to THINK in English. The definite article refers to something definite - already under discussion. So when you say “THE plot is gripping,” that means you and your conversation partner already know it’s a movie, play, cartoon, etc., so it’s redundant to say “the plot of the movie is gripping” in the real world.

So someone who really knows English would have put (of the movie) in parentheses to show that they understand about definiteness.