The object of Mr. Do! is to score as many points as possible by digging tunnels through the ground and collecting cherries. The title character, Mr. Do (a circus clown), is constantly chased by red monsters resembling small dinosaurs, and the player loses a life if Mr. Do is caught by one. The game is over when the last life is lost.
Cherries are distributed throughout the level in groups of eight. 500 bonus points are awarded if Mr. Do collects eight cherries in a row without stopping. A level is complete when all cherries are removed, all monsters are destroyed, "EXTRA" is spelled, or a diamond is found.
Mr. Do can defeat the monsters by hitting them with his bouncing "power ball" or by dropping large apples on them. While the power ball is bouncing toward a monster, Mr. Do is defenseless, leaving him vulnerable. If the ball bounces into an area where there are no monsters to hit (such as behind a fallen apple), Mr. Do cannot use it again until he has retrieved it. When the power ball hits a monster, it then reforms in Mr. Do's hands after a delay that increases with each use.
If an apple lands on a monster, or falls more than its own height, it breaks and disappears. If an apple drops a short distance and does not break, Mr. Do or the red monsters can push it off the edge of a vertical tunnel and crush one or more monsters. Mr. Do can also be crushed by a falling apple causing a loss of life.
Occasionally, the red monsters transform briefly into slower, 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. Do to collect. When it digs under an apple, it often crushes itself, other red/blue monsters, and/or Mr. Do.
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 red monster. Defeating an Alphamonster awards that letter to the player, and collecting all five letters of the word completes the level, plays a cut scene inexplicably playing the theme to Astro Boy, and awards the player an extra life. Alphamonsters eat any apples they encounter, which makes them difficult (but not impossible) to crush.
The red monsters pop up from a generator 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 red monsters, and calls out an Alphamonster and three large blue monsters. The latter can eat apples as well. The red monsters stay frozen (but still deadly) until the player either defeats the Alphamonster, defeats all three blue monsters, 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 him or her to play a free game. (This feature is relatively uncommon among arcade video games, though it is a standard feature of many pinball machines.)
Source: Wikipedia, "http://en.wikipedia.org/wiki/Mr_Do," available under the CC-BY-SA License.