Solar Imperium is a Solar Realms Elite clone. In it, you rule a solar empire. Solar Imperium's goal is to become and remain the most powerful empire. You can gain strength by buying forces, and you can gain size by colonizing planets.
You start with some planets, and a little bit of money. You are required to feed your people and army, and to pay to maintain your planets and army. If you fail to do these things, disastrous results may occur.
You are given several turns of protection, during which you cannot attack or be attacked, and cannot perform covert operations. This provides a way for smaller empires to build up their defenses and planets before they enter the real world.