"Egao" centers around a young girl named Aoi, who lives in a small, rural town overshadowed by a mysterious forest. The forest is said to be cursed, and locals avoid it, believing it to be the source of strange occurrences. Aoi, however, is drawn to the forest after discovering an old, tattered journal belonging to her late grandmother, who was rumored to have ventured into the forest and returned with a cryptic smile. The journal contains fragmented entries about a hidden world within the forest, where emotions manifest as physical entities.

Aoi’s curiosity leads her to enter the forest, where she encounters a peculiar creature named Kage, a shadow-like being who claims to be a guardian of the forest. Kage explains that the forest is a realm where human emotions, particularly suppressed ones, take on tangible forms. These manifestations, known as "Egao," can either bring joy or chaos depending on the emotions they embody. Aoi learns that her grandmother had the ability to communicate with these entities and used her power to bring peace to the forest. However, the balance has since been disrupted, causing the forest to become unstable and threatening to spill its chaos into the human world.

As Aoi explores the forest, she meets other characters who have been affected by the Egao, including Haruto, a boy who lost his ability to smile after a traumatic event, and Yuna, a girl whose overwhelming grief has created a destructive Egao that consumes everything in its path. Aoi discovers that she has inherited her grandmother’s ability to interact with the Egao and begins to help these individuals confront their emotions. Through her journey, Aoi learns about the complexities of human feelings and the importance of acknowledging and understanding them.

The narrative unfolds as Aoi and Kage work together to restore balance to the forest. They face challenges from malevolent Egao, which represent emotions like anger, jealousy, and despair, as well as from external forces seeking to exploit the forest’s power. Aoi’s growth is central to the story, as she grapples with her own unresolved emotions, including her grief over her grandmother’s death and her feelings of inadequacy. The series explores themes of emotional healing, the interconnectedness of joy and sorrow, and the resilience of the human spirit.

By the end of the series, Aoi succeeds in restoring harmony to the forest, but not without personal sacrifices. The resolution highlights the enduring impact of her journey on both the forest and the people she has helped, leaving a lasting impression of hope and renewal.