Miyuki Sawashiro and Ai Mikami have joined the cast of The Apothecary Diaries: The Late Lady's Treasure, the series' first theatrical film, Anime! Anime! reported on October 14. Sawashiro voices Ruochin (若晴), a middle-ranking consort who died in the rear palace five years earlier, and Mikami voices Inshan, who served as Ruochin's head lady-in-waiting.
The casting came with the fourth installment of a four-day run of character trailers, the "Ruochin & Inshan PV," posted to the TOHO animation YouTube channel. The film opens in Japan on December 11, 2026, and tells a completely new story, with original novel author Natsu Hyūga credited for the story concept, built around a mystery over a treasure hidden in a water city.
In the trailer, Inshan now serves another clan as head lady-in-waiting and protects a child who is treated with hostility by the clan's other children for being born to a concubine. She speaks gently of her memories of Ruochin, saying that her "elder sister" was fond of a particular flower. In another scene, she looks at Ruochin's body, returned to the consort's homeland, and murmurs "Why..." (quotes translated). Ruochin's last wish was to return to her homeland five years after her death, and the second half of the trailer shows Maomao moving closer to that mystery.
Mikami, a fan of the series before she was cast, said of the role: "Precisely because it is a work I love, I felt nervous and anxious about whether I could have a wonderful influence on it, but I faced it with everything I had" (translated). Anime! Anime! describes the film as her first appearance in a work since her NHK morning drama. Sawashiro noted in her comment that her character's name is read "Ruochin" and that she recorded together with Mikami.
The TOHO animation YouTube channel will also stream a special program from 8:00 p.m. JST on October 22, marking three years since the TV anime began and looking back at its third season. Aoi Yūki (Maomao) and Takeo Ōtsuka (Jinshi) will appear, and the program will unveil the film's main trailer, key visual and theme song artist.
Crunchyroll announced on October 8 that it had acquired North American and worldwide rights to the film, excluding Asia but including India, according to Anime News Network.