As Bub (1st player) or Bob (2nd Player) you go through 100 areas filled with enemies. In order to move on to the next area you must defeat all of the enemies by trapping them in a bubble that you shoot, then jumping on top of them or doing something that would otherwise pop a bubble. Even though it sounds easy enough, after you get through several stages the difficulty of the stages will get harder. MUCH harder. Some stages are extremely hard and seemingly impossible without 2 players.