On the first day of spring, Peter's hopes of a wonderful spring party are shattered when a mystery thief (who is a devious newcomer rat named Sammy Whiskers) secretly steals Jemima's newly laid egg and tampers with the evidence, framing Peter. As Peter fights for his innocence, things become even worse when Mr. Tod enters the equation, also going after the egg. Now, Peter must not only retrieve the egg before it hatches, but also prove his innocence and the guilt of Samuel Whiskers.