The film was written and directed by Hiroyuki Okiura and stars an ensemble cast featuring Karen Miyama, Yūka, Toshiyuki Nishida, Chō and Kōichi Yamadera.

At her mother's estate in Shio Island (汐島, Shiojima), they meet their relatives Sachio and Sae Sadahama, and Koichi, a postman and an old friend of Ikuko, who has always had a crush on her.

Three droplets from the sky enter Ikuko's estate and transform into yokai consisting of Kawa, Mame, and Iwa, the group's leader.

Momo attempts to prevent the yokai from stealing the local vegetables, but Kawa inadvertently breaks Ikuko's mirror.

After a brief argument with Ikuko, a distraught Momo leaves and talks to the yokai about her relationship with Kazuo as well as the events leading up to his fatal boating accident.

Meanwhile, the yokai realize they can escape punishment by allowing Momo and Koichi to cross over the newly completed bridge and find the doctor.

Having completed their mission to protect Momo, Iwa, Mame and Kawa transform back into the droplets and return to the sky.

[6] Yuko Hara, who is the keyboardist for the Japanese band Southern All Stars, reportedly spent 5 years working on this piece of music.

[9] A Letter to Momo made its US continental premiere at the New York International Children's Film Festival that took place March 2 to 25, 2012.

The website's consensus reads, "Sweet, sad, and visually striking, A Letter to Momo is a hand-drawn experience for animation fans to savor.

[16] Mark Schilling of The Japan Times said that Okiura "manages [the film's] transition from light to serious with the craft and assurance of a true storyteller, while firmly grounding his human and nonhuman characters in their Seto Inland Sea setting, from the narrow portside streets to the gloriously expansive view from the island's highest point," and also praised the film's hand-drawn animation.

[17] Jeannette Catsoulis of The New York Times also praised the film's look and said, "As Momo conquers her fears, averts a tragedy and finally sees the beauty of her surroundings, the movie grabs your heart with the softest of hands.

"[18] A Letter to Momo was adapted into a film manga and was serialized in the Monthly Asuka magazine, published by Kadokawa Shoten.