"Fate/kaleid liner Prisma☆Illya: Sekka no Chikai" is the fifth installment in the "Fate/kaleid liner Prisma☆Illya" series, serving as a movie adaptation that continues the story from the fourth season, "Fate/kaleid liner Prisma☆Illya 3rei!!" The narrative centers on Illyasviel von Einzbern, a young girl who becomes a magical girl and is tasked with collecting Class Cards containing heroic spirits. The movie delves deeper into the lore of the "Fate" universe, exploring the origins of the Class Cards and the parallel world known as the Ainsworth Grail War.

The story begins with Illya and her companions—Miyu Edelfelt, Rin Tohsaka, and Luvia Edelfelt—confronting the Ainsworth family, who seek to use the Holy Grail to rewrite reality. The Ainsworths, led by Darius Ainsworth, aim to create a world free from suffering by sacrificing Miyu, who is revealed to be the key to their plan. Miyu, a homunculus created by the Ainsworths, possesses immense magical power tied to the Holy Grail. As the conflict escalates, Illya must confront her own limitations and the moral complexities of the Ainsworths' goals.

The movie introduces new characters, including Beatrice Flowerchild, a powerful mage aligned with the Ainsworths, and Angelica Ainsworth, Darius's daughter and a formidable opponent. The narrative explores themes of sacrifice, family, and the consequences of altering reality. Illya's determination to protect Miyu drives the emotional core of the story, as she faces increasingly difficult challenges and uncovers the truth about Miyu's origins and the Ainsworths' true intentions.

The climax of the movie features intense battles, with Illya and her allies using their magical abilities and Class Cards to combat the Ainsworths. The resolution sets the stage for future developments in the series, leaving key questions unanswered while providing a satisfying conclusion to the immediate conflict. The movie maintains a balance between action, emotional depth, and world-building, staying true to the tone of the "Fate/kaleid liner Prisma☆Illya" series while expanding its narrative scope.