The title ostensibly refers to Grandpa Arsène – Lupin III is seeking the mysterious "Eclipse", the one treasure the old man was unable to steal (incidentally, a conceit the series has gone to more than once before). But, really, the title is more of a wink to the audience that this is "the first" CG-animated entry to the series. Delightfully, a ton of care was put into character design and animation to carry on the hand-drawn charm that the series is known for – you'll recognize facial expressions, poses, and so many little touches that make it immediately feel true to the anime. Combine that with great staging and lighting and this is a very nice movie to look at.
There's a lot of Cagliostro influence here, with a young lady joining in and Lupin acting generally more kind and pure-hearted. Director Takashi Yamazaki (who, incidentally, also wrote and directed Godzilla Minus One — making him the second Lupin director who would go on to win an Academy Award) credits that film as a major influence:
[The Castle of Cagliostro was] the first film that I first found out about Mr. Miyazaki. At the time I really had more respect for Hollywood films — it was in the 80s, so like Spielberg, etc. But then watching Miyazaki's Lupin, I realized, "Oh, you know what, in Japan, we could make high quality films too in animation." So I really understood what anime can do.The influence of Indiana Jones on Yamazaki can also be seen: it's set in the 60s and features a lot of archaeology, ex-Nazi villains, and a big "ancient technology" finale. Its flashy, modern medium doesn't let it get away from the classic Lupin feeling that makes this series great!
So – like Cagliostro – a great starting point so long as you're aware this is more of a "Nice Guy Lupin" story than most.
"How Lupin III: The First Director Takashi Yamazaki Brought A Japanese Anime Icon Into 3D", SlashFilm interview by Hoai-Tran Bui (2020)