There is absolutely no reason for the wonderful 2015 short World of Tomorrow to have needed a sequel. But we got one, and it is equally wonderful. World of Tomorrow, Episode Two: The Burden of Other People's Thoughts is a fucking miracle: it manages to succeed entirely as a follow-up to World of Tomorrow while covering absolutely none of the same ground, even though stylistically and tonally it's nearly identical. It also solidifies Hertzfeldt's status as the greatest living independent animator, as though such evidence were necessary: this is his third short film of the 2010s (the first of them was 2011's It's Such a Beautiful Day, third part of a trilogy or last third of a feature, depending on how you want to consider it), and all three of them put up a real fight as the best work of animation of the decade. For starters.

Where World of Tomorrow was globally-oriented, metaphorically considering how human beings deal with the incredible tool and soul-destroying nightmare of the internet, The Burden of Other People's Thoughts looks inward. It is all about memory, and the fear of lacking a solid identity in the present, and combating that fear by dwelling heavily in nostalgia. The science-fiction conceit this time around is "memory tourism", though it takes about a quarter of the jam-packed 23-minute film to get to that point. First, we return to little Emily Prime (Winona Mae), happily drawing pictures when a woman appears out of thin air. This is Emily-6 (Julia Pott), an unused backup for the third generation Emily who visited in the first film, now dead in the world-ending catastrophe promised by that film's narrative. With no-one around to lend her an identity, Emily-6 has felt lost and empty, and has struck on the idea of copying over the memories of Emily Prime herself. Elsewhere in all the universe, Emily-4 and Emily-7 are hunting for Emily-6, which we first see in the form of a flash-forward that is also a flash-back, depending on which character's chronology you want to follow.

But that least detail is mostly just a flourish to show off that Hertzfeldt can do whatever he goddamn well wants to, including play with time travel structure with the same perfect, jewel-like grace that he does all of his storytelling. The Burden of Other People's Thoughts is mostly about Emily-6 reflecting on the painful memories and beautiful ones she has accrued, reliving them for Emily Prime, who remains as delightfully unconcerned with all this sci-fi folderol as before (once again, the project began with Hertzfeldt recording Mae, his niece; according to the incredibly useful director's note on his Vimeo page for the film, it took a great deal more effort to wrangle her musings this time, since she was more thoughtful and deliberately creative as a five-year-old, rather than just babbling playfully as a four-year-old. If there proves to be, God willing, a World of Tomorrow, Episode Three, the insights into how human imagination evolves between ages 4 and 6 will not be least of the pleasures of the series). Once again, this is basically a monologue delivered by Pott in an emotionless monotone, explaining the history and rules of her incomprehensible future world, but this sequel is no more a direct clone than Emily-6 is, which is in no small part the point of the thing.

For one thing, Emily-6 is even more erratic than the original's clone Emily, prone to fits of shouting angrily (Pott's loud, angry monotone is one of the most delectable bits of voice acting I've heard in a long time). For another, Hertzfeldt has deepened her emotional range enormously by giving her eyebrows: his stick figures have been such extraordinarily moving emotional beings for a long time, but letting Emily-6 briefly register anger, sadness, and loss by putting little comma shapes on her forehead for just a few frames makes her as robust a feeling actor as anyone in Hertzfeldt's canon, even as much as the great Bill from It's Such a Beautiful Day.

More than anything, though, it's that The Burden of Other People's Thoughts is exploring different territory entirely: not, What is my place in history, but What is my place inside my own head? It's a more sober film than the last one: Emily Prime doesn't have so many bizarre-cute asides, and the concepts are all great deal sadder: a landscape of floating brown CGI fill with little white objects in it is a fine way to represent glimmers of hope, but Emily-6's mild observation that she hasn't produced any new glimmers of hope lately takes all the ingenuity out of it, and replaces it with solemn morbidity.

Still, this is an awfully ingenious film: having played around with computer graphics last time, Hertzfeldt goes all-out now, representing the landscape of the mind as jagged lines of three-dimensional waveforms and irregular shapes, spinning and throbbing and flowing. No question that it's the animator's most ambitious visual work yet and if World of Tomorrow can be thought of as his computer animation learning curve, akin to his early shorts in the 1990s where he worked out his style, The Burden of Other People's Thoughts is his second Rejected: the place where he explodes the medium and sees what happens when he uses it against his characters. Indeed, there is an exact digital animation version of the "crumpled paper" sequence from Rejected, when Emily-6's breakdowns cause the film to no longer be able to render; there's also an inexact version of Rejected's happy cloud people, which I cannot talk about, because it comes near the end, and it's the moment that The Burden of Other People's Thoughts becomes a true masterpiece.

It's noticeable for the first 15-odd minutes that this isn't terribly funny and that Emily Prime isn't contributing very much: both of those things change at the same time. It's also at this point that Richard Strauss's waltz suite from Der Rosenkavalier, used so potently in the first film, is brought back. I think I can say that much. And it is one of the most glorious coups du cinéma of the 2010s - if there is an argument that The Burden of Other People's Thoughts is Hertzfeldt's best film, and I think that argument exists, it's in how he and Winona Mae usher us into the film's last act, an expression of pure, cynical happiness, presented without any irony. It is, once again, an enormously dense film: raising the problem that obsessively dwelling in one's nostalgia is depressing, and then providing us with an answer to that problem, all in 23 of the most therapeutic minutes in all of animation.

The flipside is that, for this final twist to have so much power, the first 15 minutes of the film are a bit pointedly uncertain; it's visually dynamic but dramatically less so, and if there's an argument that this isn't Hertzfeldt's best film, that it merely lazes around in the top 25 films of the decade thus far rather than the top 10, it's that it doesn't grab one as quickly and violently as World of Tomorrow did. But really, God damn me if that's going to be my standard. The Burden of Other People's Thoughts is an absolute triumph, and like its predecessor deserves to be seen by absolutely everybody, animation lover or not, purely for the strength of its emotional exploration, its visual originality, and its magnificent evocation of the purity of childhood imagination.