The kids have just come back from vacation and found that the vegetables that had won a contest have rotted. Keesha is particularly distressed, not because her first prize cucumber has rotted, but because it is gone and has been replaced with a pickle. Keesha reasons that the door to the classroom was locked over vacation and Ms. Frizzle had the key. To everyone's surprise, Ms. Frizzle admits that she did do something to Keesha's cucumber. The kids put Ms. Frizzle on trial and make Liz the judge. The Friz claims that Keesha’s cucumber was turned to a pickle by a group of microbes, which she nicknames "The Mike Robe Gang," and she takes the class on a field trip to prove it.