In the game, you defend humanoids from the dreaded Manti, an alien race that loves to absorb and mutate them, and each mission gives you objectives you must fulfill in order to protect them. There are a number of space ships available, each with unique capabilities, and you can unlock more as you progress through the game. The action takes place from behind the ship you pilot, and the controls are both responsive and configurable enough for most pilots to use them effectively.
When aliens capture humanoids, if they successfully absorb them, the humanoids mutate to create a more deadly ship. It is therefore extremely important to guard, transport, and rescue humanoids as quickly as possible. Many of the missions force you to transport humanoids to reinforced positions, and if they are abducted, to rescue them by killing the alien ship and catching the falling humanoid.
You earn credits in the game, and you can use them to buy better weapons or extra lives. This provides some real incentive to complete the game, and for many, including fans of the original, the game play is compelling enough to do so.