You will start the game in the XEN headquarters.

From there, you’ll be able to access different available game modes. You can also access the menu from where you can manage and evolve XEN units, as well as the Communicator screen, where you will be able to flirt with the girls. 

During the game, you'll have to progress through missions in a tactical turn-based mechanism.

Once you complete levels, you will get different rewards.

Rewards will give you access to additional XEN units, or the ability to level up units you already own. You'll need to level up units to keep advancing through more challenging levels and unlock H-content.