From the Super Mario Wiki
In The Super Mario Bros. Super Show! episode "Jungle Fever", Mario, Luigi, Princess Toadstool and Toad venture to the Amazon Jungle in search of Sheldon, who they believe can help them defeat the evil King Koopa.
Unfortunately, before Mario, Luigi, Toad and Princess Toadstool, who were distracted by King Koopa and the Koopa Pack, could reach Sheldon's village, King Koopa and his Shy Guy minions reached it first. As Sheldon was in the process of checking several Mushroom Person patients with his assistant, Butterfingers, King Koopa had his Shy Guy's grab Sheldon, put him in a sack and run to King Koopa's base-of-operations in the Amazon Jungle.
Brought to King Koopa's camp atop a large, stone structure, Sheldon was forced by King Koopa to create him a potion that could turn Princess Toadstool to stone, under the threat of being fed to two starving Piranha Plants. Released from his sack prison, Sheldon, grabbing some supplies, began work on a potion.
After several long moments of watching Sheldon create a potion, King Koopa, becoming impatient with Sheldon, began to accuse him of stalling the creation of his potion for time. Disagreeing with King Koopa, Sheldon revealed his potion was ready, just as Princess Toadstool and Butterfingers, having found King Koopa's base due to the fumes emanating from Sheldon's potion.
Having his Shy Guy minions restrain Princess Toadstool, King Koopa proceeded to order Sheldon to douse her with his potion. Cheerfully obeying King Koopa, Sheldon, ignoring Butterfingers protestations and accusations of him being a traitor, threw a bucket full of his potion at Princess Toadstool, though not before telling her to duck.
Doing as Sheldon instructed, Princess Toadstool, as well as Butterfingers and King Koopa's Shy Guy henchmen, watched in shock as the potion, having missed Princess Toadstool, doused King Koopa; Being hit with Sheldon's potion, King Koopa proceeded to fall to the ground, scratching himself crazily, screaming of being unbearably itchy.
As King Koopa writhed and spasmed on the ground, Princess Toadstool and Butterfingers realized that Sheldon hadn't joined King Koopa and was conspiring against him the entire time he was captured. With King Koopa defeated, Princess Toadstool asked Sheldon if he could cure Mario, Luigi and Toad, who were unbelievably itchy, due to being covered in King Koopa's Koopa Strength Itching Powder; hearing this, Sheldon agreed to help Princess Toadstool in curing Mario, Luigi and Toad.
Later, Sheldon, Butterfingers and Princess Toadstool having returned to Sheldon's village, unveiled the cure to King Koopa's Koopa Strength Itching Powder to Mario, Luigi and Toad, spaghetti. As Mario, Luigi and Toad ate this spaghetti antidote, Sheldon was asked by Princess Toadstool if the spaghetti would cure Mario, Luigi and Toad's itches; Sheldon replied to Princess Toadstool's question by saying that the spaghetti wouldn't cure Mario, Luigi or Toad's itches, but it would distract them to the point that they wouldn't notice their itches.