Quartet allows one to four players to guide a set of characters through a base taken over by an army of robots. Players control either Joe (yellow), Mary (red), Lee (blue) or Edgar (green) across a number of sideways-scrolling levels. The object of the game is to advance through the level, fighting opponents that come out of portals in the walls, and eventually defeat a boss that carries the door key used to open the "exit door" for the level.
Players can find various power-ups during play, such as a jet pack that allowed characters to stay airborne, springs to jump higher, speed boots, and point bonuses. Each character has a separate characteristic weapon type, which can be upgraded by picking up a colored bouncing orb that bounced across the screen occasionally. Picking up the orb when it is a player's color gives a weapon power up, while picking up the orb when it is another player's color gives a point bonus (but deprives the other player the chance to upgrade).
Source: Wikipedia, "http://en.wikipedia.org/wiki/Quartet_(video_game)]," available under the CC-BY-SA License.