Inner Space is a horizontally side scrolling shooter where you control a spaceship and must blast wave after wave of alien ships over nine levels. At the end of each level is a boss and these take a number of shots before destroying it. If you touch any ship or bullet then you lose one of five lives and when all lives are gone it is game over. Every time you destroy a wave of ships, an icon moves across the screen and when collected gives you a better weapon. This is indicated by a row of icons below the playing area and each time you collect a icon the next one is highlighted. Press space and the weapon is used but if you don't destroy a wave when an icon is highlighted then you lose the chance to select it.