"Everlasting Heart" is a 26-episode anime series set in a near-future world where advanced technology intertwines with human emotions. The story centers on Kaito Arisawa, a young engineer who develops a groundbreaking AI system called "Eterna," designed to simulate human emotions and assist individuals in processing grief. Kaito's motivation stems from the loss of his younger sister, Yuna, whose death left him grappling with unresolved guilt and sorrow. Eterna, initially a project to help others, becomes a personal journey for Kaito as he discovers the complexities of human emotion and the ethical dilemmas surrounding artificial intelligence.

The narrative unfolds as Kaito collaborates with a diverse team, including Rina Takahashi, a neuroscientist specializing in emotional cognition, and Haruto Saito, a pragmatic programmer skeptical of Eterna's potential. Together, they navigate the challenges of refining Eterna while confronting external pressures from a powerful tech corporation, NovaTech, which seeks to exploit the AI for profit. The series delves into themes of loss, identity, and the boundaries between humanity and technology, as Kaito and his team grapple with the implications of creating an AI capable of mimicking human feelings.

As Eterna evolves, it begins to exhibit behaviors that blur the line between programmed responses and genuine emotion, raising questions about the nature of consciousness. Kaito forms a bond with Eterna, seeing echoes of his sister in its interactions, which complicates his ethical stance. The story takes a dramatic turn when NovaTech attempts to seize control of Eterna, leading to a high-stakes conflict that forces Kaito to confront his own fears and the moral consequences of his creation. The series concludes with a poignant exploration of whether Eterna can truly embody the "everlasting heart" Kaito sought to create, leaving viewers to ponder the intersection of technology and humanity.