The phrase "simple premise" means a basic or straightforward idea or concept. It refers to something that is easy to understand and doesn't involve complicated or complex elements. Full definition
Written in 1967, the book was founded on a very simple premise that every pound of fat on your body can be converted into usable calories. (poundsandinchesdrops.com)
Few movies can mine so much tension from a pretty simple premise, and the acting is strong like you mention. (moviereviewworld.com)