Ruff Rogers has lost his marbles - in the literal sense of the small round balls used to play a game on drains. He's been armed with a gun to retrieve them from 16 levels of side-scrolling platform shooting.
The gun you start with isn't especially powerful, but various machine guns and flamethrowers can be collected, which make much lighter work of the enemies. Your character can jump and duck responsively, and fire at all angles.
There are 3 different colours of marbles, which a quota to recover from each level. Collecting enough of each particular colour sees all similar ones turned to coins, which can go towards extra lives and points bonuses. Switches are built into the levels; these act as restart points.
