Well, the title pretty much sums it up. Mario Is Missing! Bowser has kidnapped Mario to prevent him from stopping his newest scheme. The Koopa King has unleashed a plot to wreak havoc across the globe by stealing artifacts from historic sites around the world.
Luigi must travel from city to city throughout the world, with help from Yoshi, recovering the stolen items from thieving Koopas, and searching for Mario to finally foil Bowser's dastardly deeds!
The graphics in Mario Is Missing seem a bit primitive by our standards, but I think it's mainly because we're used to seeing Luigi, Yoshi and the gang, in cartoon worlds rather than a pixelated representation of our world. In any case it doesn't take away from the gameplay, which is fun and quite educational.