Diddy will fight his legal case to the bitter end ... because his lawyer says a plea deal won't fly with his client.

Diddy's attorney, Marc Agnifilo, appears in TMZ Studios' new documentary -- "The Downfall of Diddy: The Indictment" -- available to stream now on Tubi ... and, he says his client will fight this case not just for himself, but for others in a similar position.

Agnifilo says Diddy says he's innocent ... so, he sees no reason to take a plea deal -- even if prosecutors offer it.

And, MA says Diddy isn't just fighting for himself ... he's using his case to inspire others who believe they're facing government persecution -- wanting to inspire them to fight back when the feds bring the hammer down.

Diddy's family's on his mind as well, Marc says ... he says Diddy wants to prove to his fam he's the same man they've always known.

As we told you ... Agnifilo also says Diddy's being prosecuted by the feds because he's a successful Black man.

We dive into these topics and more in "The Downfall of Diddy: The Indictment" ... chronicling his indictment, defense and a lot more.