Plot:

You crash land on an alien planet called Elcore and discover the ruins of a long dead civilisation. You discover the reason there civilisation ended was because they tried to create portals that could transfer people from one place to another, and they succeeded. But they did not plan for what would happen on the other side. The other side of the portal opened and closed in random places in the universe. This allowed hordes of random aliens to swarm in and destroy their civilisation. You are now stuck on this planet and need to defend against the oncoming hordes of unknown aliens.

Game play:

This is a tower defence game so you will build towers along path. You start out with a small amount of towers and as you destroy aliens you get scraps from their weapons and can use them to create your own versions of their weapons. Occasionally you will have a friendly group of aliens come through the portal. These alien will teach you to combine your weapons with alien scraps to make more efficient guns. The game ends when you ether get destroyed or you last 40 round. When you reach round 40 Rescue will arrive and you win.

Image: https://en.wikipedia.org/wiki/Stargate#/media/File:Sg1stargatefront.jpg