In the course of events in the story, a group of pirates besiege the University and begin making threats. One member of the student body knows these pirates very well, and has a bone to pick with them. Although they constitute a real menace, and have no mercy to show the innocent students, they are no match for the unexpected magical gifts of some of the students, and as a result, they are turned into mice.
Which promptly infest the campus and start wreaking the kind of miniature havoc that a large number of small creatures are well able to wreak. Pirates aren't good; Mice aren't good, but deliver us all from the twin threat of pirate mice!