Ernie and Judy go to New York City to intercept an evil wolf serum, but their mission is interrupted when they run into Emma and Zuri ("Jessie") at a Halloween party. Meanwhile, Marissa and K.C. throw their own Halloween party at home, and K.C. dresses up as a spy to impress a new boy at school.

When a former foe needs the Coopers' help, the family has to decide if they can face the consequences of aiding the enemy.