The show is set in 1969 when the world is under attack by aliens known as invaders. However, this war is waged in secret by an organisation called AEGIS. This organisation uses unique indviduals who can open 'gates allowing them access to special powers. These people are called gatekeepers. One such gatekeeper is Ukiya Shun.
The plot is pretty much what you would expect but it is done in a very interesting way. The characters are very well written and have some excellent voice talent. The animation is vibrant but not childish or over done. The plot lines are also excellently portrayed and especially towards the end of the series take on a very interesting direction.
Just for reference I am not talking about gatekeepers 21 here which this guide counts as a second series but is actually a spin-off and does already have it's own guide on the site. It is also quite inferior to the original show.