The game begins with your tank escaping from a drop boat onto the beach, shortly before an enemy bomber flies overhead and destroys the boat. From then on, you are on your own. Your mission is to make it from that beach to the enemy's headquarters deep in enemy-held territory.
In order to do this, you will have to follow certain paths which end with a boss fight. Once you clear certain areas, you may find that the path splits up, and you must decide which path to follow. There are a number of different paths that you can take in order to reach the headquarters. The path that you follow is up to you. You can always see your current position by looking at the map on the sub-screen.