Mr. Junkett (Sherman Hemsley), a vehicle maintenance inspector, was so displeased when he inspected the bus that he decides to take it to the junkyard to be crushed. Ms. Frizzle and the class have to do something to save it. Naturally, they have decided to shrink and go inside its engine. All the problems seem to be caused by peanut butter, as Mr. Junkett was eating a peanut butter sandwich while he was inspecting it.