TMZ has uncovered the storyline for the music video starring Levi Johnston that pissed off Bristol Palin so much -- that she considered it the "final straw" in her engagement.
As we previously reported, Bristol claimed she was furious when she found out Levi was doing a video "mocking my family" ... and dumped him soon after.
Now, we've obtained the "script" for the video in question -- for a song called "After Love" by Brittani Senser -- in which Levi plays a man who is seemingly threatened by his lover's mother ... in order to get him to get out of her daughter's life.
But here's the part that may have REALLY pissed off Bristol -- in the video, the girl's mother uses a police officer to arrest Levi and drag him to an empty parking lot ... where she confronts him and forces him to open a mysterious manila envelope.
Levi opens the package -- and his face turns "pale" at its contents. The mother then makes Levi call up her daughter and break things off ... with the insinuation that she would expose something if he refused.
Levi has previously denied the video was "mocking" the Palins ...
but what do you think?