The show is set more than 30 years after the original gatekeepers show and the war with the invaders is still being waged. AEGIS still exists but is a shadow of it's former self. The show follows Ayane Isuzu one of the new generation Gatekeepers as she struggles to defeat the invaders and recruit fellow gatekeepers to her cause.
This is a solid show but lacks something when compared to the original. The characters are darker and more troubled which although makes them more believable also makes them harder to like and leaves you feeling less involved with them. The animation is also less vibrant and gives the show a darker tinge.
All in all i think my fondness for the original has clouded my opinion on this spin-off.