Steven Universe has been steeped in music from its start, and Steven Universe: The Movie takes it to the next level as a full-out, feature-length musical.

The film, which premieres Labor Day on Cartoon Network, comes complete with songwork by series creator Rebecca Sugar along with stars like Aimee Mann, Chance the Rapper, Gallant, and, of course, Estelle, the voice of Garnet. Series composers Avi and Surasshu and series writer and storyboarder Jeff Liu also worked to create more than 10 songs — which span multiple genres — to pack into the movie's 82-minute runtime. For Sugar, it's the realization of a long-held dream.

"Years and years ago, I thought that it could be a possibility for us to do a movie musical before there was an idea for it. I started thinking about it right away because that's just a complete lifelong dream to get to do," Sugar told TV Guide.

Sugar and her team tried to create a movie musical episode during the run of the show, but it "didn't fit inside the story that we were telling," she said. Instead, they conceptualized a movie as a way to explore the lives of the characters after the conclusion of the planned five-season story. "We were going to sort of keep growing the story even after these characters feel like they're finished, which became the story of the story of the film," Sugar said.

Of course, Steven is no stranger to musicals. The Season 3 episode "Mr. Greg" is about as old-fashioned a musical as you can get, with song and dance propelling much of the narrative and giving voice to characters' feelings in a way that previous episodes could only devote a single song to. The movie feels like a fully-formed version of the impulses you see in "Mr. Greg," and that's entirely by design. For Sugar, there wasn't any other way to tell this particular story.

"We wanted to approach it really differently than we had approached the series and do things that make sense in the language of musicals that we might not be able to do in a format that's not a full musical," Sugar said.

That includes doing front credits — something Sugar jokes that "no one's been willing to tolerate since the '70s" — and really thinking about how the show's characters fit into certain musical traditions and then representing that on the screen.

Early in the movie, we're treated to a recap of the series, told by White Diamond (Christine Ebersole) and done in a storybook style that anyone familiar with animated Disney films that predate the Disney Renassiance period will recognize. "The Diamonds are supposed to be indicative of a sort of golden age animation fairy tale language," Sugar explained. "So to do their opening and to do their sequences, we really wanted to treat them like an old musical. I really wanted to make it feel classic in the way that [the Diamonds] feel classic, and [play with] the incongruity of that and the harmony of that with the other characters — just the way that the characters play off of each other — because they're so different from each other. I wanted to showcase that through music and through visuals."

Steven Universe: The Movie Photo: Cartoon Network

That incongruity carries over in other ways, too, specifically with the movie's antagonist, whose name we won't reveal here. Sugar said that writing the antagonist's introductory song, which occurs about 10 minutes into the movie, was a difficult task because she was enjoying herself too much. Its style is "so indulgent to me," Sugar said, "that I was just beet-red while I was writing it because I was having way too much fun, and it's like it shouldn't be allowed."

In one of the movie's opening numbers, Steven sings, "Here we in the future, and it's bright/ Nothing to fear, no one to fight/ I can't believe we've come so far/ Happily ever after, here we are." When the song is reprised at the end of the film, it serves as the culmination of five seasons of work by Sugar and her team. "We were pulling out all the stops with everything we'd ever learned from 162 episodes, plus everything we had figured out over the course of working on the movie," she said.

The movie and its music do feel like they could be marking the end for Steven Universe, but Steven's final lyrics in the film might be enough to give fans hope for more stories from the world: "Happily we'll face whatever comes our way, and after we might do it all again."

Steven Universe: The Movie premieres Monday, Sept. 2 at 6 p.m.