All great heroes and villains are supposed to be diametrically opposed on something and that’s why Steven and Spinel worked very well against each other, a really simple point of contention with a million complications

The movie started out with that recap of the series told as a storybook, open and closed, beginning to end. Steven is two years older, they’re building a peaceful hope for all gems on Earth, the Diamonds are now like his overbearing aunts who hug too long, the song “Here We Are In The Future” presents this stage as the endpoint - the future is here, this IS the ending, the story has finished. Steven repeatedly refers to his ‘happily ever after’ from now on, a state of harmony he put in the work to achieve, and now gets to enjoy.

And then Spinel comes in, and I could draw a villain comparison from her to Undertale’s Flowey - someone who knows you are the protagonist, who won’t let the story end because they still want to be included, and will ‘reset’ you to force the narrative to continue for their benefit.

Spinel resents that the story took place without her. Moreover, she’s opposed to Steven’s proclamation that the world is fixed now, forever, because the moment she heard that was the moment she realised how broken she was. She would’ve wanted to be in the story, and she would’ve been on the good guys’ side, but nobody included her so she’ll find them and erase their character development, so that she can redo the story and insert herself in it. Her villainy only extends as far as ‘you finished it wrong, you left me behind, start over and put me in the narrative’. After she resets the CGs and Steven’s gem she keeps laughing as Steven returns the blow and resets her too, implying maybe that it was her plan to force them back to stage one and erase her memories after making sure she was now present as the story repeats.

Then Steven has to recreate everything he’s learned about his family to bring back their true selves - from the connections they’ve made to their heartbreak. There’s a very consistent message that your ‘true self’ isn’t you without your bad experiences, unaffected by them, that you’re a whole person because of them.

But the funny thing about the clash over whether or not the story is finished is that Steven is wrong. When Steven says ‘this point of happily ever after is where the story stops’, Spinel hears ‘you’re stuck here and can’t develop past your trauma’. She sets out to prove him wrong because she hasn’t yet recovered. She hasn’t done her character arc yet. She’s missed everything. Steven’s side of the argument is essentially to deny that Steven Universe, the cartoon, will continue, and Spinel’s argument is that she’ll make it continue until she gets closure - and of course any continuation of their story will involve the character development that Spinel craves and that Steven explicitly says he’s done having.

The movie is a metanarrative about characters who are vaguely aware of their continued existence only when there are more problems to be solved, as recognised by the language they use, like the line “I’ve heard the story over and over again” referring to their own plot, and Peridot’s “but my character development” joke. When Steven laments that he still doesn’t have his powers back despite reliving every terrible thing that’s ever happened to him, he realises his mistake; that you can’t inhabit a preserved moment of reality forever, and things will continue to change, but the ability to do so is the entire reason he as a half-gem hybrid exists. And the change he embodies is why he infuriates Spinel and why she wants what he is - “When you change you change for the better, when I change, I change for the worse”. She wants to recover from abandonment and stop unhealthy ways of expressing her grief, and doesn’t think herself capable.

Which is why when Steven, still in denial, says she can shut down the drill and they’ll ‘forget it ever happened’, that triggers a relapse. Forgetting it ever happened was her unhealthy plan to get her memories wiped and plant herself in the story. Forgetting it ever happened means discarding the growth she was just able to make and reliving the realisation that nobody in the universe cared about her. In saying that, he reveals he hasn’t figured out that she needs the continuation to be able to heal past this. Spinel’s character development is opposed to Steven’s desire for an unchanging ending– and he realises he’s wrong and only then is able to assure Spinel in the final song, Change: “You can make it different, you can make it right / You can make an effort, starting with tonight” which effectively gives her permission, from the main character, to alter his world and his story, to have the time, starting now, to work on herself as part of the fiction he’s established - for someone who sees herself as a broken character, inflicting herself and her unfinished arc on someone who finished theirs, to be allowed to exist and told that her unhappiness is welcome among people who have adjusted better from theirs.

The tone of the ending is all about effort. Steven’s realisation is that he’ll always be putting in work towards making the universe safe and happy. Spinel’s realisation is that she CAN be a good person if she wants to, and that decision can only be made by her. Steven’s final act of renewing the life of the Earth from poison by going around kissing patches of it is a metaphor for the constant work one must put in to heal from toxicity. Spinel’s final act of leaving Steven, after seeing him affectionately greet his family, is a literal undertaking of the same task, as she says outright: “I’ve got work to do. Friendship isn’t going to be easy for me. I’m gonna have to work at it. You make me want to try.” She thought what she wanted was to take revenge on the owner of the gem that wronged her, but what she really needed was to stop fixating on the owner of that gem entirely. She recognises that she has a journey ahead and it would be better taken away from Steven, and she cannot insert herself into his life, which is already full of loved ones. She wasn’t there when they all bonded and she can’t reset that. All she can do now is take control of her future.

Which she does. If anything, this movie is about there being no single state of being a good, fair person. It’s an effort you undertake for life, and you can always try to be better at it. You’ll never be done growing. Happiness might be temporary, but anguish will be temporary too. That’s life for you.