G.O.D: Mezame Yo to Yobu Koe ga Kikoe (1996)

The year is 1999. A boy named Gen lives together with his mother in a small Japanese city. One day he goes to the nearby mountains and discovers an underground cave. In this cave, he encounters a strange bat-like creature guarding what appears to be a huge diamond. When Gen approaches the diamond, the floor crumbles, and all he can see is a group of aliens descending on the Earth… Gen is awakened by a man in a military uniform. He quickly gets up, and notices a calendar. The year is 2009! Ten years have passed, of which there is no recollection in Gen’s memory. The world has changed a lot during those years. Aliens have invaded the Earth, many cities are destroyed, and he finds himself amidst the members of an underground resistance movement. Now he has to travel all over the world, help the humans in their struggle, and discover the secret of the invasion.