Doc and Marty go to Ancient Peru in the year 1532 to find a cure for Marty's athletes' foot. The cure comes in the form of an extinct frog — the Bufo Marinus — which emits an acid from its skin. Doc and Marty meet up with the vile conquistador Biffando de la Tanén and his goons. Biffando releases them and follows them at a distance, to the lost City of Gold. Marty and Doc end up being captured by Incan natives and are put in a large pit with hundreds of Bufo Marinus frogs. With that many frogs, Doc says that he and Marty will "melt like the Wicked Witch of the West." Using a tuning fork, Doc makes the frogs go crazy and it looks like Marty can "control" the frogs. The Incans release Doc and Marty and stop Biffando from stealing any gold. Doc and Marty take a couple of frogs home to reestab