For a movie that had a run-time of 1 hour and 30-minutes, it covered a lot of ground. For people who have been following the T.V. series in its entirety, its a gift to the fans. And for people who never seen Steven Universe and are new to it, they'll understand and love the characters and for what they are.
The crewniverse did a wonderful job when it comes to both story and music, blending seamlessly into the plot of the movie with care. The story is shown as followed: a new enemy shows up to attack and destroy plant earth, challenges are brought to the table for each character, and some repressing and emotional feeling are brought that can be relatable and understood to any person.
Hope it's vague enough.
I hope you get to watch the movie when you can. I will admit, the main antagonist is annoying, but like any character from the show, you'll get to know more of them soon than later.