Earthlings have not yet had time to properly explore the cosmos, and the threat from it is already quite real. Several generations of mankind sent their messages to outer space. Apparently they reached the goal and one of the rational races intercepted them. But alien creatures were much higher in development and do not want to share their technologies. They just want to turn the Earth into their own colony, one of the many that they already have. To avoid enslavement, you must protect yourself. It was decided to create a shield around the planet. It consists of a small segment that needs to be rotated, setting where the threat comes from in Defend Earth.