Steven Universe: The Movie begins two years after the main series, with Steven and the Crystal Gems enjoying an era of peace. This is shattered by Spinel, who arrives on Earth with a massive injector poised to destroy all organic life. She wields a Gem Rejuvenator, a scythe that she uses on Garnet, Amethyst, and Pearl, reverting them to their original, pre-character development states and erasing their memories. Steven manages to use the weapon on Spinel, which also resets her to her original form: a goofy, playful companion.
The central conflict forces Steven to help his friends remember who they are. Amethyst regains her memory through Steven's unconditional acceptance, Pearl remembers her independence after being inspired by the fusion of Steven and Greg (Steg), and Ruby and Sapphire remember their love for each other, re-fusing into Garnet. Through this process, Steven discovers Spinel's backstory: she was created to be Pink Diamond's playmate but was abandoned in a garden for 6,000 years when Pink left to start her colony on Earth. Spinel only learned of Pink's fate after seeing Steven's universal broadcast.
After Steven helps Spinel regain her memories to shut down the injector, she feels used and her rage returns, believing Steven will abandon her just like his mother did. In the final confrontation, Steven realizes his own powers have been weak because he has been resistant to the idea of change, clinging to a fantasy of a "happily ever after." Embracing the fact that he will always have more work to do, his powers return, and he is able to defeat Spinel not with force, but with genuine empathy. He validates her pain and shows her she can change. The film ends with the Diamonds arriving on Earth. Instead of taking Steven, they are charmed by Spinel and offer her a new home, which she accepts, finally finding a place where she is wanted.