"In a Pickle" is the seventh episode of season two and the twentieth episode overall of the animated children's series The Magic School Bus. It originally aired on October 21st, 1995
As the class returns from vacation, Keesha is horrified to find that her prize winning cucumber has been changed in to a pickle. When Ralphie walks into the classroom, Keesha begins to accuse him of stealing it (much to Ralphie's digust) until Ms. Frizzle, who is eating a pickle, walks in. When Keesha enters a storage full of pickles, she begins to suspect the teacher. Ms. Frizzle admits that the cucumber is "gone, but not missing", telling Keesha that the pickle used to be a cucumber. Keesha, however, doesn't buy it. So she attempts to eat it, only to get squelched by Arnold, who states that there would be no evidence that Keesha's cucumber has changed to a pickle if she were to eat it. When Keesha begins to interrogate the teacher, Arnold stops her and tells Ms. Frizzle to remain silence and dismiss the class. This gives Ms. Frizzle an idea to go on trial with the rest of the class.
The class holds up a court in the classroom, with Keesha and Ms. Frizzle as the defendants and Liz as the judge. Keesha starts off by showing everyone the difference between a pickle and a cucumber. Arnold acts as the teacher's defense attorney, shocked that Keesha would even suggest she did it. Ms. Frizzle admits she had something to do with the missing cucumber, but it was the "Mike Robe Gang" that was responsible. She also adds that the "Mike Robe Gang" are invisible because they are too small to be seen with the naked eye. She gets Arnold to suggest a field trip, and they learn first hand about microbes, which is what changed the cucumber in to a pickle. As the class hop onto the bus, Ms. Frizzle transforms it into a mini blimp and flies into the classroom.
In the classroom, the bus lands onto the teacher's desk and the class walk out to find their lost material inside the "Lost and Found" box. They come across a flower with multiplying yeast called Ascomycota (commonly known as ascomycetes), which is found everywhere. Carlos jokingly states that "I rather find it on your seat than my seat". Arnold then comes to a conclusion that if anyone could see it when they're tiny, they're a member of the Mike Robes Gang. Keesha, however, is still not convinced. So the class and Ms. Frizzle hop onto the mini-blimp-transformed bus and head to the vegetable garden in Keesha's deference. But Keesha changes her mind and decides that the bus should land onto Liz instead.
So Ms. Frizzle complies and lands the bus on Liz's nose, where they view a number of microbes on her scaly skin. Liz, oblivious to the bus' appearance, thinks it is just a pest and flicks it off, sending them in the hamster's water bowl. They view strange things in the water, the fat, juicy ones being amoeba and the ones with the appearances of hot dogs are paramecium, more members of the Mike Robe Gang. Despite this, Keesha is still adamant of not believing Ms. Frizzle's explanation of the cucumber changing into a pickle since the water the hamster was drinking didn't change like the cucumber. So Arnold suggest that the class should go to his rotten 3rd prize tomato.
On the tomato, Arnold unintentionally gives Ms. Frizzle an idea to shrink even smaller and the class dress into their anti-fungus suits offered to them by Ms. Frizzle. The class exit the bus and watch the fungus fall apart. they start sprouting roots as they land onto the tomato juice. Even though they see more members of the Mike Robe Gang, Keesha is still unconvinced about the cucumber changing into a pickle, much to Arnold's annoyance. Just then, the tomato juice erupts like a volcano and the class spills along with it inside a jar of water.
They end up in the jar with the pickle and find a number of microbes inside. They find that the fungus are being destroyed by the pickle bacteria on the cucumber. Outside of the jar, Liz gives the rotten tomato a taste and spits it out in disgust, causing another cucumber to fall into the jar. The class finally learn that the pickle bacteria is what kept the cucumber from rotting like the tomato, thus they get rid of the fungi. They also learn that the pickle bacteria are part of the Mike Robe Gang as well, prompting them to realize that Ms. Frizzle was talking about microbes. Keesha finally learns that the pickle in her jar is really her cucumber transformed into a pickle thanks to the pickle bacteria.
Back inside the classroom, Ms. Frizzle is declared innocent and Keesha thanks Ms. Frizzle for putting her cucumber in the jar to keep it from rotting like Wanda's carrot and Arnold's tomato. Tim also provides a drawing of the cucumber, turning it into a prize pickle. Ms. Frizzle concludes that, "Cucumber come and cucumbers go, but pickles go on forever" (which she derived from her great uncle). The case is dismissed and the class laughs as Liz's gavel falls apart.
- Ms. Frizzle's prisoner number is a reference to Jean Valjean.
- It is revealed that Tim draws a picture of the scene featuring Ms. Frizzle, Dorothy Ann and Keesha during the trial when Ms. Frizzle talks about the "Mike Robe Gang".
- Arnold recalled the events in "Wet All Over" when he said, "I hope we're not back on the water cycle".