In 1971 and 1972, Hayao Miyazaki began his directorial career on the Lupin III television series, alongside his "senior partner" Isao Takahata. After that series was cancelled after 23 episodes, Miyazaki took his first solo flight, directing the 1972 pilot film, Yuki's Sun (Yuki no Taiyo).
In Japan, a "pilot film" is similar to a demo reel, or "proof of concept." Its purpose is to demonstrate to possible investors or producers how a proposed film or television series will look. It will usually contain a montage of short scenes, paired with an overarching narrative. There are a number of famous anime pilot films, particularly the 1969 Lupin III pilot created by Yasuo Otsuka and Masaaki Oosumi, the 1984 Nemo pilot directed by Yoshifumi Kondo, and the 1987 Nemo pilot directed by Osamu Dezaki. Some of them, as Samuel Jackson would say, get picked up and become television series; most others don't get picked up and become nothing. Yuki's Sun is one of the ones that became nothing.
Yuki's Sun was based on a Japanese manga comic by Chiba Tetsuya, which was serialized in Weekly Shoujo Friend in 1963. It was common then, as now, for manga comics to be spun into television. Unfortunately, the project was unable to win support with networks or producers (perhaps still feeling the sting at the "failed" Lupin show?), and that was the end of that.
What does this pilot film tell us about Hayao Miyazaki at this point in his career? Yuki' Sun shows a growing confidence in his skills as a director, as seen in the layouts and shot compositions, the skillful use of timing in the various "scenes." I personally suspect that the latter Lupin episodes were solely his work, especially the series finale, which compares very favorably to the 1980 Lupin Series Two finale, "Farewell, Beloved Lupin."
It is also important to understand the degree of freedom given animators at Toei Doga, where Miyazaki began his career. Under the Toei system, the director was little more than an office manager, coordinating various teams together, making sure everything fits together, but without any real creative input. The studio bosses and the animators were in charge. And while it's common for key animators in America to be in charge of a specific character - one person animates Aladdin, another person animates Genie, and so on - at Toei, key animators were in charge of scenes. They were responsible for all the key drawings for that scene, the proper timing. Also, they may be responsible for creating the storyboards and image boards; this was the case for Toei's 1969 Puss in Boots and 1971 Animal Treasure Island.
(As a point of comparison, Isao Takahata was really the first auteur director, with The Great Adventure of Horus, Prince of the Sun. He established the director as the main creative force, and although not himself an animator, as per the Toei system, Paku-san held final say over every aspect of the production, and strict control over every scene and cut.)
Because of this, Hayao Miyazaki had all the necessary skills to become a director. He was, essentially, the "director" of his key animated scenes in Puss in Boots, Animal Treasure Island, and Ali Baba and the 40 Thieves. And his unique role in scene design and layout, most famously with Horus and Heidi and Marco, brought a true artist's sense of framing and composition. It's this quality that I especially enjoy about his films. He creates animated movies like graphic novels, not "cartoons." Western animators could learn much by studying his work.
Yuki's Sun feels like a transitional work, and was created shortly after the infamous Pippi Longstockings debacle. You can see how Isao Takahata, Hayao Miyazaki, Yoichi Kotabe, and the rest of their crew were moving towards literature, towards pastoral naturalism, away from comic book escapism and cliffhanger serials. The two Panda Kopanda short films would follow in '72 and '73, and after that, the steady march towards Heidi in '74. For that, this pilot film provides some insights into this most important period.
Yuki's Sun was only aired, partially, on Japanese TV in 2001, as part of promotion of Miyazaki's Spirited Away (which would become his biggest blockbuster success). It was finally released in full as part of the 2014 Hayao Miyazaki Blu-Ray Box, and that is the version we see today. The subtitles are fan-created, and do not appear on the official Japanese release.