So, I'm not sure if this is correct, I might be wrong and this is just speculation. But I think this is the thing that inspired Lupin III The First, not The Castle of Cagliostro, as the director had said. And I feel Eternal Mermaid is way more consistent, plotwise, in comparison. Both movies are great, don't get me wrong, and I love them both. It has some interesting internal mythos that manages to suspend your belief (unfortunately some Lupin stuff can't pull this off, it's rare but it happens), the characters are at their best here, specially Fujiko. It touches on Lupin's relation to hisgrandfather, a theme I really enjoy when it shows up. Sympathetic "episode" related characters who are very cute and bounce off well with Lupin.

It gives Goemon a rival. There are some nice moments with Jigen (I do think the duo was a biiit underused, but with a cast this big, it's not rare for this to happen and I kinda already expect it from these 1h30 specials.

I think the only major criticism I have for this special is it's portrayal of Zenigata, I know each character changes a bit from iteration to iteration, but even if I, for example, didn't enjoy too much Full Hard Boiled And Harasser Zenigata from The Woman Called Mine Fujiko, I will say that he at least was very competent. Zenigata in Eternal Mermaid just kinda stumbles along the plot, being there because He Has To. I didn't hate him in it, I'm always endeared by how much he believes in Lupin's integrity, but I felt he deserved more credit.

I guess that's all, overall a very good watch, I recommend it to anyone who likes Lupin III.