[NOTE: It goes without saying, but if you haven't at least seen the B't X TV series then the following synopsis features spoilers!]
Teppei Takamiya is still on the path towards the center of The Area, where the Machine Empire's Main Tower is located, so that he can rescue his brother, robotics genius Kotaro Takamiya. After going through B't X's death, and rebirth at the hands of Spirit General Hokuto & B't Max, Teppei and X are now a solid duo and have even gotten the remaining Spirit Generals, Foh, Lon, & Hokuto, to question whether their leader, the Machine Emperor, is truly a god or if he's a devil. Meanwhile, Kotaro is still stuck in UnderHell, where Warden Marcello accidentally dropped a vial that contained a piece of B't Raphaello that was supposed to go to Kotaro for research. Even though Demon General Myslim & B't Larenya have been defated time is running out before Raphaello completes its transformation, which will mean the end of the world, and if they have any chance of stopping it Karen, Empire traitor & Teppei's mentor, has to discover the Great Light that Kamui Island might be hiding deep inside.
B't X Neo wastes no time in getting straight to the story, which continues on with Demon General Quatro & B't Loresso. If there's one major difference between the TV series & Neo it's that the other major characters truly get their moments to shine. For example, Quatro is a former friend of Foh's who wants the Spirit General dead, and though Teppei does get involved during the fight in the end this is all about Foh. Neo also delves into Hokuto's past, Major Aramis' (the person who kidnapped Kotaro) former life, the reason why Misha & Nasha, the Machine Emperor's personal agents, joined the empire, and even Lon starts to feel that he might have been nothing more than a simple pawn to the empire. Hell, even Metal Face, the first person to fight Teppei, & Marcello get some bit of development & redemption when all is said and done. It's great to see all of the major characters become relateable to the viewer and become someone you can understand... All the more so for Hokuto, since he barely appeared in the TV series.
In terms of adapting the manga, Neo is a bit odd. Normally when an anime diverges from its manga source it stops doing so at a specific point, but most of what came before it is fairly accurate to the manga. B't X Neo, on the other hand, is only mostly accurate for the first three episodes, i.e the Quatro fight, followed by episodes 4-9 being mostly divergent material with some small bits of manga adaptation mixed in. This results in some manga scenes being adapted later than they would have been as well as three characters being portrayed differently from the manga. First there's Demon General Maim, who in the manga is related to Hokuto's backstory but in the anime is simply another guy for Teppei to fight, though the way Maim is handled still works well enough. Then there's Aramis, whose major change is that in the anime she was a legendary warrior for the empire before the Spirit Generals were around. Compared to how what happens to Aramis in the manga Neo does win out a little bit in terms of portrayals, but at the same time when her past is revealed Aramis essentially leaves the anime, only to come back for a short appearance in episode 13. The last change is in Juggler, a masked man in a clown/pierrot outfit who acts as a leader of the forces that are supposed to kill Teppei & the traitors. Though Juggler's overall attitude is the same as in the manga his actions and backstory are handled pretty differently.
The last five episodes end up being completely divergent from the manga, and what Neo ends off with is an epic battle against a demonic being that, while not being written by the man himself, definitely gives off the feel of a Kurumada title. I in no way claim to have watched everything out there, but I still haven't seen that many anime out there with an ending that is handled as epically & tightly-paced as B't X Neo. Tight pace? Yeah, those definitely exist. Epic feel? Others have that, too. But Neo has a great combination of the two that doesn't come around that often. Oddly enough, though, Neo does have some scenes which are very similar to how the manga would handle them later on, especially the scene when Karen approaches the Great Light after finding it, which happens in Volume 15 of the manga. Considering that the anime only adapts into Volume 8 of the 16-volume manga, one would think that either Kurumada told TMS of the base ideas he had for the end of the manga or possibly Kurumada was inspired by the direction Neo went. If someone was to ask me which ending, Neo or the manga, I preferred I would honestly have to say that both are worth seeing. Though there are similarities they both go in pretty different directions overall.
Though this is a sequel to the TV series that is produced by the same animation studio, B't X Neo does have a slightly different staff behind it. Gone is Mamoru Hamatsu from the director's chair, and in his place is Hajime Kamegaki, director of series like Fushigi Yugi & Kenichi The Mightiest Disciple, with writing headed up by Yoshiyuki Suga, who worked on titles like Dragon Ball, DBZ, One Piece, the B't X TV series, and Saint Seiya (specifically the original TV series, Movies 1 & 3, & the Lost Canvas anime). Going off of a resume like that Kamegaki & Suga certainly know how to handle stories that can go into "epic" territory, so having the two for Neo is certainly a great idea. Hideyuki Motohashi is still behind the character designs, which results in the characters having that Kurumada look, but still keeping a style that isn't that of Araki & Himeno. Akira Senju is gone from the music composer position, with Fumitaka Anzai, who I talked about in my Combustible Campus Guardress review, handling the music duties here. Though Anzai does mix it up a little bit, like Senju did, Anzai's focus in Neo is clearly about being orchestral and symphonic, which definitely helps add to the epic styling the OVA gets in its last episodes. The opening theme is "A Piece of the Sun" by Masaaki Endoh, the legendary anison singer who now is a big part of JAM Project. Unlike the TV series' opening theme, which went for more of a Asian influence, Neo's opening theme is more of a hot-blooded song that gets you ready for the action and story that's coming... Very much like the other opening themes to Kurumada animes. The ending theme is "Towa no Sono Saki ~You Are the Best Buddies~" by Masaaki Endoh, and this song is clearly the gem of the soundtrack; it's upbeat and positive message about keeping your "buddies" close to you is truly the best message one can get from B't X, and the fact that an instrumental version of this song plays whenever Teppei & X get into their "kicking ass mode" shows that even TMS & Anzai knew how special this song was. The last episode features an insert song, "Hitomi wa Ikusen no Mado" by Aki Hata, that starts at the final climax and plays all the way through the final credits, and this song is probably one of the most fitting final songs that played for an anime's climax. Considering how the story ends it just would have been almost inappropriate for "Towa no Sono Saki" to be the last song heard here.
All the voices from the TV series return for this OVA, so Nobuyuki Hiyama is still Teppei, Jin Horikawa is still B't X, Keiji Fujiwara is still Lon, etc. I did not give any real credit to Hokuto's seiyuu, Osamu Sakuda (who rarely does anime, with Hokuto being his largest role), in my review of the TV series but Sakuda really pulls out a great performance in Neo, making Hokuto sound believable as the knowledgeable "Master Doc" but also tough and ready to fight. Quatro is voiced by Toshihiko Seki (Duo Maxwell in Gundam Wing, Noboru Takizawa in Blazing Transfer Student, & Sanzo in Saiyuki), and he easily brings out the crazed vengeance that's on Quatro's mind. Shinichiro Miki also pulls out a very good Maim, which makes his reduced focus all the more a shame. In all honesty, B't X has a very large cast, mainly due to the fact that each B't is an extra character to consider, but the cast here is excellently done as a whole.
The B't X TV series was a really good adaptation of the manga, & B't X Neo not only continues that quality but actually surpasses it by diverging into its own ending that really delivers in terms of being an epic ending the likes of which Masami Kurumada is known for. B't X was the title that got me into Masami Kurumada's works, and when I first saw this ending I was floored and amazed, as American animation doesn't normally go into the direction that anime can go. Now, all these years later, I still feel that Neo's ending stands up excellently and was a great culmination to everything that had happened before it. Essentially, the climax itself is almost 4 episodes long, and even the parts that come before it are handled in a way that makes it so that you can't truly tell what is going to happen next. Looking at the B't X anime's licensing history it's sad to see that ADV never worked on it after licensing it back in 2002, and it's even sadder that Illumitoon's attempt was so poorly done. The chances of this series getting licensed again are extremely slim, but B't X as a whole is another great example of why Masami Kurumada shouldn't just be recognized for Saint Seiya, as he has proven multiple times, both before & after doing Seiya, that he knows how to deliver great action titles with stories & characters that grab your attention.