"Trinity Seven: Eternal Library & Alchemic Girl" is a feature-length anime film that serves as a continuation of the "Trinity Seven" series. The story revolves around Arata Kasuga, a high school student who becomes a Demon Lord candidate after a mysterious event known as the "Breakdown Phenomenon" causes his hometown to disappear and his cousin, Hijiri Kasuga, to vanish. Arata enrolls at the Royal Biblia Academy, a magical institution, to uncover the truth behind these events and to find Hijiri.

At the academy, Arata is surrounded by seven powerful female mages known as the Trinity Seven, each specializing in different forms of magic: Lieselotte Sherlock, the head of the library and a master of grimoire magic; Arin Kannazuki, a swordswoman with lightning-based abilities; Akio Fudo, a ninja skilled in shadow magic; Levi Kazama, a demon summoner; Yui Kurata, a shrine maiden with purification powers; Mira Yamana, a gunner who uses alchemy; and Lilith Asami, the academy's headmaster and a master of gravity magic. Together, they assist Arata in his quest while navigating their own personal challenges and rivalries.

The film's plot centers on the discovery of the "Eternal Library," a mystical realm that holds infinite knowledge and power. Arata and the Trinity Seven are drawn into a conflict involving Selina, an alchemic girl who seeks to use the library's power to rewrite reality. Selina's motivations are tied to her tragic past and her desire to create a world without suffering. As Arata and his companions confront Selina, they must grapple with the ethical implications of altering reality and the potential consequences of wielding such immense power.

Throughout the narrative, themes of sacrifice, redemption, and the pursuit of truth are explored. Arata's determination to save Hijiri and his growing bond with the Trinity Seven drive the story forward, culminating in a climactic battle that tests their resolve and unity. The film expands on the series' lore, delving deeper into the nature of magic, the responsibilities of a Demon Lord, and the interconnected fates of its characters. The resolution ties together key plot threads while leaving room for future developments in the "Trinity Seven" universe.