"Mori no Andō" is a science fiction anime series set in a near-future Japan where society has been transformed by the emergence of a mysterious phenomenon known as the "Forest." The Forest is a vast, ever-expanding area of dense vegetation that has overtaken much of the country, rendering large swaths of land uninhabitable and isolating communities. Within the Forest, strange occurrences and anomalies are commonplace, including the appearance of bizarre creatures and the distortion of natural laws. The government has established a specialized unit called the Forest Exploration Team to investigate and contain the Forest's growth, but their efforts have yielded limited success.

The protagonist, Andō, is a young man who lives in a small town on the edge of the Forest. His life takes a dramatic turn when his younger sister, Yuna, mysteriously disappears into the Forest during an expedition. Determined to find her, Andō joins the Forest Exploration Team, despite his lack of experience. As he ventures deeper into the Forest, he encounters other individuals who have been affected by its enigmatic nature, including a scientist named Dr. Kanzaki, who is researching the Forest's origins, and a skilled but enigmatic explorer named Rina, who has her own reasons for delving into the Forest's depths.

The narrative unfolds as Andō and his companions navigate the Forest's dangers, uncovering fragments of its history and the truth behind its existence. They discover that the Forest is not merely a natural phenomenon but a sentient entity with its own agenda. The series explores themes of human resilience, the consequences of environmental exploitation, and the blurred line between science and the supernatural. As Andō gets closer to finding Yuna, he must confront the moral dilemmas posed by the Forest's influence and the sacrifices required to protect those he cares about. The story builds toward a climactic revelation about the Forest's purpose and its connection to humanity's future.