The Doctor Is In... Over His Head
Mario is rushing to the Mushroom Kingdom General Hospital, because he believes there is a plumbing emergency there. He has enlisted Princess Toadstool to carry his tools for him (because Luigi is busy), but upon arriving, the team finds out there's no plumbing emergency, much to Mario's disappointment. But the scatterbrained Dr. Waldo Bloom remarks that there is indeed an emergency here - he needs a new research assistant, and he's decided to give Mario that position. Mario's not keen on the idea (or on being called Dr. Mario), but the Princess insists.
Dr. Bloom shows Mario and the Princess through the hospital, showing off the room where they cure Mushroom People and enemies alike (they make enemies sign a contract to be good, though, as the Goomba in the room shows, it doesn't always work). Dr. Bloom then brings them to his laboratory, where he explains that he's recently discovered Viruses that have been multiplying ever since. He has placed the original specimens under a giant microscope and the multiples in a jar. Suddenly the original Viruses break loose from their container and land inside the giant microscope. Mario uses his plunger to open it up, releasing the now human-sized Viruses, who introduce themselves as Chill (blue), Fever (red), and Weird (yellow). After inflicting their respective sicknesses on Mario, the Princess, and Dr. Bloom, the three break their smaller kin loose and escape through a window.
Mario takes off after the Viruses, but not before Dr. Bloom lends him his coat (which turns Mario's hair brown). He soon catches up to them and tries fending them off. Eventually he discovers a supply of vitamins that Dr. Bloom had stored in the coat's pockets and finds that stacking Vitamins of a certain color on the same-colored Viruses eliminates them. He then proceeds to take out the Viruses. Afterwards, he returns to the hospital, only to find that Princess Toadstool, now in a nurse uniform, has suggested that Dr. Bloom take up golf. Dr. Bloom will not return until he's finished his game, but Mario notes that this could take a very long time (Bloom has made 456 strokes and is still on the first hole). Peach suggests that Mario stick around as the Mushroom Kingdom's doctor in addition to being a plumber, which he eventually agrees to do.