They start with new revelations regarding what's thought of Gate Keepers' powers, Invaders' origins, and who is masterminding this new wave of corruption. To go with all that, the campy nature of the TV series is gone, and replaced with a gritty, dark feeling. Even during the daylight scenes, there is this oppressive feeling that permeates everything. It was a breath of fresh air in a stagnant plot.
Overall, the odd personalities of the new Gate Keepers, mixed with the evolution of powers and abilities on both sides provide some excellent surprises. I'd reccommend this to anyone who thinks originality is bleeding from anime.
Gate Keepers 21 starts out in the year 2001 where the Invaders are still very plentiful and actually set in present Tokyo. Compared to the TV series, Gate Keepers 21 is a lot darker in mood, tone, and animation than its original counterpart. The protagonist of this OAV series is Ayane Isuzu, who uses cell phones and technology to harness the power of various gate powers. She tends to stay away from using her own gate power, the Gate of the Wind (the same Gate power that belonged to Ukiya Shun of the TV series). I don't think it's too hard to guess their relation. Because of her past (which we never really find out), Ayane isolates herself and battles the Invaders alone but tried to enlist the help of Miu, a Gate of the Jump user from her school. Miu is somewhat cowardous but she has her reasons to not want to fight the Invaders. The same bad guys from the TV series are back but are headed by a mysterious blonde girl instead of Kageyama. Currently, Kageyama is actually on the side of good, heading AEGIS.
Final Note: You can expect the same amazing quality of digital animation and sound. I actually did enjoy this series quite a bit. This series is more about human relationships and the feelings, doubts, and fears that people have in their minds. In that, it was really good. However, they didn't really touch up on their pasts or the 'futures' of the TV series cast. They touched it but never really went into it which made me crave more. Hopefully there will be just one more to explain the questions that have been brought up through the past two series.