The new sinister Gem, named Spinel, is revealed to have a personal vendetta against Steven and the Crystal Gems. She was left behind on Homeworld for thousands of years by Pink Diamond, who she believed was her best friend, until Pink Diamond abandoned her and started a new life on Earth as Steven. Spinel's abandonment led to her being filled with anger and a desire for revenge, resulting in her creating the giant drill to destroy all life on Earth as a way of getting back at Pink Diamond (or Steven).
As the countdown ticks down, Steven and the Gems struggle to find a way to stop the drill before it's too late. Along the way, they encounter a new ally in the form of a human scientist named Dr. Sebastian, who offers his expertise in stopping the drill. They also face a number of challenges, both physically and emotionally, including an intense battle against Spinel, who proves to be a formidable foe.
Throughout the movie, there are also a number of musical numbers, including a show-stopping performance by Spinel in which she sings a song about her anger and desire for revenge. Meanwhile, Steven and the Gems sing songs about the importance of love, friendship, and teamwork, which help them stay focused on their mission and give them the strength they need to save the planet.
In the end, after a series of intense battles and emotional revelations, Steven and the Gems successfully stop the drill, saving all life on Earth. They also manage to help Spinel find closure and come to terms with her past, ultimately leading to her redemption and a new understanding of what it means to be a true friend.