You can take the easy route and tweak the existing Plex Movies scraper, or you can take the (very slightly) harder route and install a totally new scraper that pulls ratings from IMDb and Rotten Tomatoes by way of The Open Movie Database. If you want more accurate ratings pulled from a broader pool of movie reviewers, there are two ways you can go about it. This is how you end up with so many movies in Plex that have 3/5 stars-many of the rated movies on The Movie Database only have a few reviews and average 5-6 out of 10, which translates to 3/5 in the Plex interface. While we love The Movie Database, and it’s great for getting most metadata (like artwork and direction/cast information), the rating side of things simply isn’t as well developed the ratings on sites like IMDb and Rotten Tomatoes. The deal is that Plex movie media agent, the little bit of software that pulls metadata about your movies from internet database, uses The Movie Database by default. In fact, except for exceptionally well-known and loved movies (like The Godfather) or really awful movies, very few movies have a 5 star or 1 star rating. If you’ve taken a look at the star ratings of the movies in your Plex library, you may have noticed something a little peculiar: there’s a remarkable number of 3/5 star movies.
Where Plex Gets Movie Ratings (and Why You Should Tweak Things)