By AI generated it will mean something similar to the prompt system used for Stable Diffusion except the output will be a movie of 5 minutes or more in length with multiple scenes or angles with the same characters.

Believable will mean a movie that does not necessarily need to fool the average viewer but will allow the easy suspension of belief while watching it (i.e. an animated Pixar style movie is considered believable but something rapidly morphing between different images would not).

The tools must be available to the average person, either by running them on your own local machine or using a startup or similar hosted service (which can be a paid service but must be priced at a level that makes it available without too much difficulty)