Kingdom Hearts III continues with the same story as the previous games where the power of friendship is used to defeat the looming darkness. Evil cartoon characters fight against Square Enix and Disney characters in RPG and combat games/missions.
The key to rescuing the universe
Kingdom Hearts III is similar to the other games where a character wields a key that blocks an invasion by dark forces. You fight your way through RPG areas with your cartoon comrades and use the key to seal the world off against evil cartoon characters. There is a running story line between all the Kingdom Hearts games, but it takes little more than a bit of plot reading on Wikipedia to catch up on what has happened previously. The game is less about overarching narrative and is more about enjoying time with your favorite Final Fantasy and Disney characters. If you played the first South Park Stick Of Truth game (Fractured But Whole is not worth playing), then part of the pleasure comes from the cut scenes in equal measures to the pleasure you feel when playing. If you are a fan of Disney cartoon characters, then playing Kingdom Hearts III almost feels like you are hanging out with them on one of their adventures. The characters interact with each other like they are friends during the cutscenes, which is not something you get to enjoy when you watch their cartoons and movies.
It is about the experience
As mentioned earlier, there are games you can play for the experience rather than for the gameplay and story. Stick of Truth did it, and some of the Final Fantasy did it perfectly. Kingdom Hearts has never been about ground-breaking gameplay or a stunning story, it has always been about the in-the-moment experience that you get when you spend time with your favorite TV and movie characters. Play for long enough, and you will notice a few glaring absences such as Final Fantasy characters who look realistically human and older Final Fantasy characters such as those seen in the 2016 games. The developers may have felt that realistic-looking characters may look odd when allied with cartoon characters, (though it worked well for Who Framed Roger Rabbit).