Fin and Fifi are nervous before their first day of school. However, once they arrive and see their excited classmates, the brother and sister fish begin to relax and discover that they enjoy learning about interesting subjects and going on fun undersea adventures.

Lenny has relocated his family back to the small town where he and his friends grew up. This time around, the grown ups are the ones learning lessons from their kids on a day notoriously full of surprises—the last day of school.