"The Feast of Amrita" is a supernatural mystery anime that revolves around a secluded island where a ritualistic feast is held every 50 years. The island, known as Amrita, is shrouded in secrecy, and its inhabitants are bound by ancient traditions. The story begins with a group of outsiders invited to the island under the pretense of participating in a cultural celebration. However, they soon discover that the feast is a deadly ritual designed to sustain the island's immortality at the cost of human lives.

The protagonist, a young journalist named Haruto, arrives on the island to investigate its enigmatic history. Alongside him are several other guests, each harboring their own secrets and motivations. Among them are Aya, a historian with a personal connection to the island, and Kaito, a skeptical scientist who dismisses the supernatural claims surrounding Amrita. As the feast commences, the guests are drawn into a series of increasingly bizarre and dangerous events, uncovering the island's dark past and the true nature of the ritual.

The narrative unfolds through a series of flashbacks and revelations, gradually piecing together the island's origins and the role of the feast. The island's immortality is tied to a mythical entity known as the "Amrita," which demands sacrifices to maintain its power. The guests must navigate a web of deceit, betrayal, and supernatural phenomena as they struggle to survive and uncover the truth. The story explores themes of mortality, sacrifice, and the consequences of seeking eternal life.

Key characters include the island's enigmatic leader, Shizuka, who serves as the primary antagonist. Shizuka is deeply committed to preserving the island's traditions, even at the cost of human lives. Her motivations are rooted in a tragic past, adding complexity to her character. Another significant figure is Ren, a mysterious islander who aids the guests but has his own hidden agenda. Ren's knowledge of the island's secrets makes him both an ally and a potential threat.

As the story progresses, the guests are forced to confront their own fears and desires, with some succumbing to the island's influence while others fight to escape. The climax reveals the true extent of the Amrita's power and the sacrifices required to break free from its grip. The ending leaves room for interpretation, with lingering questions about the nature of immortality and the cost of defying fate.