In Mario Brothers Plumbing, Mario and Luigi are dressed like doctors, doing what they can to keep Mama Mario's dying basil bush alive. Unfortunately for them, Mario concludes that things are grim, and that the only thing he and Luigi can do now is hope and pray. Luigi doesn't want to have to tell Mama that her basil bush is dying, and Mario states that they need a miracle.
To their surprise, the doorbell rings, and a man who calls himself Dr. Toby enters. Dr. Toby claims that he can tell whenever a plant is in distress, and knew to come over beforehand. He refers to his ability as either a gift, or a curse, and presents Mario and Luigi with his new Super Plant Growth Formula, but warns them that it is in the experimental phase, and that they should follow the instructions carefully. Mario and Luigi thank Dr. Toby, and he leaves.
As Luigi begins to read the instructions, he notes that only a small amount of formula should be used. Desperate to revive the basil bush, Mario disregards the instructions and sprinkles a massive amount of the formula on the plant
Later on, Mario calls Dr. Toby to inform him that his plant formula worked... too well. He explains that as a result, the formula transformed the basil bush into a large Plant Monster, which is holding Luigi hostage in an embrace. Relaying advice from Dr. Toby over the phone, Mario tells Luigi to ask the plant monster to let him go, but it doesn't work. Mario then tells Luigi to try tickling the monster, but that doesn't work either, along with teasing it and dancing. Mario then tells Luigi to run, and it works, but the Plant Monster chases both brothers around the basement, and eventually catches them.
Being held down by the Plant Monster, Mario and Luigi keep themselves occupied by playing chess until Dr. Toby returns. When he arrives, Dr. Toby is thrilled to see that his formula works, and announces that he's created an antidote to the formula which will shrink the plant back to its original form. As he prepares to do so, the Plant Monster attacks him, but Mario physically restrains him as Dr. Toby sprays the antidote.
Mario is mildly disappointed, for he enjoyed the fight he had with the monster ("What'd you do that for, Doc? I had him right where I wanted him!"). Dr. Toby then mentions that he developed a fast-acting Super Yeast Formula that Mario and Luigi could use for their pizza dough, but they quickly refuse the offer. But for now, the basil bush is healthy again, and Luigi notes that Mama will be happy now.