Once upon a time, there was a 2012 anime called Sword Art Online. It was pretty good and the franchise is still running, but its most iconic storyline is still its original one. This was about a community of virtual gamers getting locked inside a game by its mad designer, who'd built VR headsets called "NerveGear" that killed you if you removed them or logged out. You'd die if anyone interfered with your NerveGear. You'd die if your game character died. The goal to escape was to complete all 100 levels of a very difficult game while never losing a life, because that one life was all anyone had.
Incidentally, I've only just discovered that Sword Art Online has a child series, Accel World, written by the same author and set in the same universe, but a generation or two later. Those light novels have been running since 2009 and they got a 26-episode anime series in 2012, plus a 2016 movie. I'll return to those another day.
The problem with that original storyline is that the author (Reki Kawahara) galloped through it too fast. The first Sword Art Online novel was written for a competition with a page limit. He overran that and ended up publishing it as a web novel, but it's still a very abbreviated version of what it could have been. 100 floors, each potentially an epic in itself, in which the players lived for two years. An entire world's worth of characters. Eventually, Kawahara went back and rebooted/rewrote it all as a new light novel series where he'd fleshed everything out to its natural length, called Sword Art Online: Progressive. It's just an expanded version of the original, but there are differences and technically they're different continuities. (Asuka gets together with Kirito much earlier in Progressive.)
Progressive now has its own anime adaptation, although not as a TV series. So far, it's two movies.
(a) The first movie (2021) is the equivalent of episodes 1-2 of the TV series, told from Asuna's viewpoint rather than Kirito's.
(b) The second movie (2022) tells an all-new story set on Aincrad's 5th floor. This goes between the second and third episodes of the TV series, which had already jumped to floor 11 at the start of ep.3.
Today, I'm talking about the 2021 movie... and it's pretty good. Afterwards, I went back to compare with the original series and was startled by the movie's fidelity. At least half of the movie is all-new, with scenes and characters we've never seen before, but there are no contradictions. Common scenes will be word-for-word identical. The same characters, same dialogue, same character designs. Everyone's doing the same things in the same deathworld. We're just seeing things that had previously been offscreen. Characters who deliver a single line in a single shot will do so in both versions, e.g. Ayano Keiko's cameo. Even the two versions' visual choices are identical (albeit better animated here), e.g. those red warnings flashing across the sky before a slime avatar bleeds forth to say "you're all screwed".
Illfang the Kobold Lord (movie version) looks a bit more badass, while the TV series spends more time explaining how things work and heading off potential headscratchers. Basically, though, you could watch them back to back and it would feel like one story. Not even a reboot, but a single narrative. You only need to watch one version of any shared scene, though, for which you'd probably choose the movie for its art and animation quality.
If you want to experience everything, I'd suggest beginning with the movie. Watch that from start to finish. It's a far more complete narrative, compared with which the TV episodes in hindsight feel like an abridgement of it. 2012 episode 1 is still worth watching, though, because its Kirito-Klein story isn't in the movie. Theoretically, it's identical to Asuna-Mito. They're a gender-flipped copy-paste. Kirito and Mito are both mentor beta-testers who already know Aincrad and the tricks for getting ahead. Asuna and Klein are newbies who've befriended a mentor and will get taken under their wings.
In practice, though, they're very different. Kirito and Klein are both gamers and their reactions feel truer to a gamer's natural responses. Asuna's a newbie. She's clueless. She's never played a game like this before and she doesn't even know online etiquette. She gets traumatised, she needs help with the absolute basics and there's no equivalent in the TV series to the development of her relationship with Mito. (Especially how they part. Asuna's character growth is the film's story. We see a lot of her real life at the beginning, including her cold, pushy mother and a best friend.) All that's good and I like these variant explorations of the same basic template.
You can skip the second half of 2012 ep.1 and most of ep.2, though, which are the same as the movie.
There's only one thing here I didn't buy. Misumi gives Asuna her trademark game hairstyle... but in real life, so it's unclear how it got transferred to her game avatar (which was based on a photo of herself with plain hair).
It's a good film. Sword Art Online was already a strong series, but this is a more emotionally rich introduction to it. It's still hitting the familiar beats, e.g. "Rather than lock myself inside an inn and slowly rot away, I'd rather stay myself until the last moment." Here, though, we know where Asuna's coming from there. It even adds a little flavour to Kirito's characterisation, e.g. his dorky moment when failing to look cool when sheathing his swords.
The film also does something refreshing in not killing the Token New Character who's original to this film and is Asuna's best friend we've never seen before. I'm looking forward to the second Progressive film.