Breakthrough is played on a 5x9 board. Each player has ten pieces which start in the rows nearest them. The object is to get a piece to the row farthest away from you. On a turn a piece can move any distance diagonally as long as it does not pass over any piece or land on an opponent's piece.
The fourth and seventh rows of the board are "wall" rows, and pieces which reach the opposing wall cannot move forward. The only way to get a piece across the wall (necessary to win) is via explosion. Each space has a maximum capacity (depending on which column it is in), and when you get that many of your pieces in a space it explodes, sending the pieces to orthogonally adjacent spaces. Opposing pieces can be captured this way, and chain reactions can occur.