The city has been attacked by the rebels, but the base have spotted some locations, and sent you to eliminate them. you have to defeat the rebels, and finish the mission.
Controls: Use the left mouse button to shoot (hold press to do rapid fire), press down arrow button to duck or hide, and space button to reload the bullets.
A context menu should be shown when right-clicking or, on touch devices, long-pressing in the game. This menu especially allows to play in full screen and control the volume.