Nancy's friends are sad because the class hamster has gone missing. Poor Nibbles. But then Katie brings in a bunny named Bun Franklin to be the new class pet! Everyone is thrilled, and now they can began preparing for the Spring Fair.

But soon enough someone starts wrecking the class's Spring Fair projects, and everyone thinks it must be Bun Franklin! Mrs. Reynolds is afraid he can't stay. But Nancy is sure it's not Bun—how can she prove it? It takes a lot of clue hunting and sleuthing for Nancy to realize that what she's uncovered just may solve two mysteries at once!