After the events of the original anime series, Mustang is transported to an alternate reality, where he is now a military officer in Germany. He is haunted by the memories of his time in Amestris and particularly misses Hawkeye.
The film shows Mustang struggling with his new life and longing for his old friends, especially Hawkeye. He eventually learns that Hawkeye has also been transported to this alternate reality and is searching for him. He reunites with Hawkeye near the end of the film, and they finally share a moment of peace after all they've been through.