"Gekijōban Fate/kaleid liner Prisma Illya: Sekka no Chikai" is a film adaptation of the "Fate/kaleid liner Prisma Illya" series, serving as a continuation of the fourth season, "Fate/kaleid liner Prisma Illya 3rei!!" The story centers on Illyasviel von Einzbern, a young girl who, alongside her friends, has been tasked with collecting Class Cards containing the powers of Heroic Spirits to prevent a dimensional collapse. The film delves deeper into the lore of the "Fate" universe, exploring the origins of the Ainsworth family and their connection to the parallel world known as the "World of Eternity."

The narrative begins with Illya and her companions—Miyu Edelfelt, Rin Tohsaka, and Luvia Edelfelt—facing the aftermath of their battle against the Ainsworths. Miyu, a key figure in the conflict, is revealed to be a homunculus created by the Ainsworths to serve as a vessel for the Holy Grail. The Ainsworths, led by Julian Ainsworth, seek to use Miyu's power to rewrite reality and save their dying world. As the group uncovers the truth about Miyu's origins, they are drawn into a confrontation with the Ainsworths' forces, including powerful Heroic Spirits summoned through the Class Cards.

The film explores themes of sacrifice, identity, and the consequences of tampering with reality. Illya, determined to protect Miyu, must confront her own limitations and the moral complexities of the situation. The story also introduces new characters, such as Beatrice Flowerchild, a servant of the Ainsworths, and further develops the dynamics between Illya and Miyu, emphasizing their bond and shared struggles. The climax involves a high-stakes battle against the Ainsworths, culminating in a dramatic resolution that sets the stage for future developments in the series.

The film maintains the series' blend of magical girl elements with the intricate lore of the "Fate" franchise, offering a balance of action, emotional depth, and world-building. It serves as a pivotal entry in the "Prisma Illya" storyline, advancing the overarching narrative while providing insight into the characters' motivations and the larger conflicts at play.