Seatbelts, everyone! The Magic School Bus is back and ready to ride again. Professor Frizzle's kid sister Fiona might be the new teacher at Walkerville Elementary, but she's got the same old motto: take chances, make mistakes, and get messy! Carlos desperately wants a classroom pet and he's got the perfect idea-- a rat! But when Carlos can't stop sneezing, the class has to figure out what's going on, so they head straight to the source--his body!