This sad story will, by itself, make you depressed.
What nuance does this "by itself" add?
The latter, sir.
I am not a teacher.
"By itself" means on its own, without any help from anything else.
Perhaps it means, "Without the need for any mood music to cue your emotions".
Or knowing the background of the people, or having a similar situation happen to you, or be in a down mood yourself to start with.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
So it means "This story alone will make you sad?”
The music is emotive. The cinematography really captures the imagination. However, the story alone will move you to tears".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.