In the game, the hero Opi is traveling through time with four of his friends when they are captured by the villain Zenzombie. He travels through various eras in time (modern, samurai, future, World War II and prehistory), collecting treasure to open doors to the next era, and kills enemies by digging holes and burying the enemies. Every 15 of the game's 60 levels, Opi rescues one of his friends. Levels 30 and 60 are boss levels where an enormous enemy floats around the screen who can kill Opi with a single touch.
Source: Wikipedia, "Cratermaze", available under the CC-BY-SA License.