Gunple: Gunman's Proof

Two meteors fell on Earth on a small island near the west coast of North America. In the middle of the island there is a small town governed by a sheriff. Strange things started to happen like monsters suddenly attacking people while leaving behind a message that said demiseed.

One of the aliens introduces himself as Zero, the Space Sheriff, and explains that an intergalactic criminal named Demi has escaped from justice and is hiding somewhere on the island, being the cause of all the troubles lately.