Hey Arnold! stars nine-year-old Arnold (voiced by Lane Toran; Phillip Van Dyke; Spencer Klein; Alex D. Linz and Mason Vale Cotton) and his neighborhood friends: Gerald (voiced by Jamil Walker Smith and Benjamin Flores Jr.), a street-smart character who generally serves as the leader of the group, and Helga (Francesca Marie Smith), a girl who bullies Arnold in order to hide the fact that she is in love with him. Bartlett drew inspiration from people he grew up with when creating the characters for the show.
Arnold lives with his eccentric but loving paternal grandparents, Phil (Dan Castellaneta) and Gertrude (Tress MacNeille), proprietors of the Sunset Arms boarding house, in the fictional city of Hillwood. In each episode, he often helps a schoolmate or boarding home tenant in solving a personal problem or encounters a predicament of his own. Many episodes involve urban legends usually told by Gerald, such as superheroes or the headless horsemen.
Other characters include students and faculty at P.S. 118, Arnold's school, and citizens of Hillwood. Certain episodes focus on the lives of supporting characters, such as the tenants of the boarding house that Arnold's grandparents own.