Alien Soldier is a side-scrolling action game in which the player takes the role of Epsilon-Eagle, the former leader of the Scarlet mutant group. The mutants are now lead by the evil Xi-Tiger who had turned them to terrorism, and one of their goals is to kill Epsilon-Eagle...

At the start of the game the player chooses four out of six weapons - each with special advantages or disadvantages against certain enemies. As mutant Epsilon-Eagle also has other special powers, e.g. teleporting across the screen or (with full health) morphing into a powerful phoenix destroying everything in his way.

The 25 levels consist of short side-scrolling sequences and a few boss fights. In fact the bosses take on the majority of the game and the side-scrolling sequences are mostly used to give the player opportunity to collect ammo boxes. Bosses have their own patterns and weaknesses.