The object of Mr. Du! is to score as many points as possible by digging tunnels through the ground and collecting cherries.
Cherries are distributed throughout the level in groups of eight. 500 bonus points are awarded if Mr. Du collects eight cherries in a row without stopping. A level is complete either when all cherries are removed, all creeps are destroyed, "EXTRA" is spelled, or a diamond is found.
Mr. Du can defeat creeps by hitting them with his bouncing "power ball" or by dropping large apples on them. While the power ball is bouncing toward a creep, Mr. Du is defenseless. If the ball bounces into an area where there are no creeps to hit (such as behind a fallen apple), Mr. Du cannot use it again until he has retrieved it. When the power ball hits a creep, it then reforms in Mr. Du's hands after a delay that increases with each use.
Mr. Du or the creeps can push an apple off the edge of a vertical tunnel and crush one or more creeps. If an apple falls more than its own height, it breaks and disappears. Mr. Du can also be crushed by a falling apple, causing a loss of life.
Occasionally, the creeps transform briefly into more powerful multicolored monsters that can tunnel through the ground. If one of these digs through a cherry, it leaves fewer cherries (and fewer points) for Mr. Du to collect. When it digs under an apple, it often crushes itself, other creeps, and/or Mr. Du.
Each time the score passes a certain threshold during play (5000 points), a letter from the word "EXTRA" appears on the playfield as an Alphamonster, and the player can defeat or be defeated by this monster in the same way as a creep. Defeating an Alphamonster awards that letter to the player, and collecting all five letters of the word completes the level, plays a cut scene playing the theme to Astro Boy, and awards the player an extra life. Alphamonsters attempt to eat any apples they encounter, which makes them difficult (but not impossible) to crush.
The creeps spawn at the center of the screen. After they have all appeared, the generator will turn into a food item; picking this up scores bonus points, freezes all the creeps, and calls out an Alphamonster and three large blue monsters. The latter can eat apples as well. The creeps stay frozen (but still deadly) until the player either defeats all three blue monsters, defeats the Alphamonster (in which case any remaining blue monsters are turned into apples), loses a life, or completes the stage.
Rarely, dropping an apple will reveal a diamond which, if collected within about 15 seconds, completes the level and awards a bonus credit to the player (as well as 8000 points), allowing the player to play a free game.