The Japanese and European versions are nearly identical, but the American version changes the game's plot by adding an opening text that establishes the game to be set inside a giant alien life-form which is infected by a strain of bacteria. Stages that feature starfield backgrounds were changed to the web background from Stage 1 to maintain consistency with the organic setting of the plot. The power-ups are also given different names, with the "Speed-Up" becoming "Hyper Speed", the "Missile" becoming the "Destruct Missile", the "Ripple Laser" becoming the "Pulse Laser" and "Force Field" becoming the "Shield". Narration is added to the beginning of each stage, detailing the area of the alien's body which the player is currently inhabiting such as "Enter stomach muscle zone" or "Bio-mechanical brain attack".