The Mad Doctor is a recurring villain who appears in a few Data East titles as the main antagonist.
In his first appearance, the Mad Doctor spreads various monsters and aliens around the world, so the two Tumble Poppers fight the monsters around the world, before facing the Mad Doctor in the moon.
Diet Go Go
The Mad Doctor once again shows up, this time spreading junk food to make everyone fat, so two kids go after him to stop his plans.
Joe & Mac Returns
This time the Mad Doctor travels to prehistoric times, where he kidnaps all cavewomen for a Neanderthal village, so the two heroes Joe & Mac go after him to rescue the girls. He fights on an large robot made of wood very similar to the one he uses in Tumble Pop.
The Mad Doctor steals a pice of a treasure map from Johnny and kidnaps his girlfriend, holding her for ransom in exchange for the remaining map pieces, so Johnny joins with his friend Tom, as the two go after the doctor and fight his goons.