The player controls the gapper, a sprite that travels along the lines of a grid of rectangles, changing them from blue to red. A rectangle completely surrounded by red lines becomes lit, and the player scores points. He or she progresses to the next level by lighting all the rectangles on the grid. At higher levels, the game plays faster, the grids contain more and smaller rectangles, and many grids have irregular shapes with several corners.
The gapper is pursued by the seeker, a second sprite that moves along the grid, always along the most direct route towards the gapper. If the seeker overtakes the gapper, the gapper is destroyed, and the player restarts the level with one less life. The gapper has one defensive mechanism: it can leave a temporary "gap" in the grid that the seeker cannot traverse, allowing the gapper to put distance between itself and the seeker.