Professor Mariarti is, as the game title suggests, a mad scientist. Not all of his experiments worked out the way they should and now he needs to shut down the power in five of his labs (the first four can be tackled in any order). This plays out as a standard platformer: Mariarti jumps over gaps between platforms and dodges enemies (he can take a few hits before dying) until he reaches the end of the level, i.e. the off switch.
At the start of the game he only throw wrenches to defend himself, but during the game he finds coins scattered around the labs. Those can be used to buy more advanced weapons like a laser cannon. However, killed enemies respawn after a short time. To proceed he also needs to solve a few puzzles by collecting items and bringing them to the correct place. There are also a number of switch puzzles.