After learning that Joey, a new boy at his school has no friends, Arnold takes it upon himself to befriend him and take him home to meet his family. While there, he convinces Willis to become a "big brother" to Joey. As a result, everyone at the Drummond household take such a shine to Joey, that Arnold becomes jealous and decides to run away.

A substitute teacher makes life hard for Arnold and Dudley, so they try to make things difficult for him in return. When one of their pranks leads to Arnold getting hit by accident, they go to the principal and tell him that it was done on purpose, which gets the teacher in trouble and nearly gets him fired until Arnold and Dudley come clean about what really happened.