The school is doing the play 'Romeo and Juliet'. Arnold lands the role as Romeo so Helga will do anything to beat out other girls to get the part of Juliet, so that she can kiss Arnold in the final scene.

Arnold asks Grandpa what really happened with his parents, and he finds out that they disappeared after they left the country on a plane, although they might still be alive.

Arnold is sick and tired of everybody performing their April Fool jokes on him (escpecially Helga), so he tries one of his own, but it backfires when it temporarily blinds Helga.

When corrupt developer Schek threatens to buy the run-down urban neighborhood where a fourth grader Arnold lives, and now Arnold and his cool-as-ice best pal Gerald are on the biggest mission ever to save their neighborhood as they go up against a powerful industrialist who invades the city and buys everything in sight so he can knock it down and erect a huge "mall-plex." It's up to Arnold and Gerald to save their neighborhood before it's too late and they'll have to move somewhere else. But with the help of of a superhero and a mysterious deep-voiced stranger, Arnold and Gerald will need to recover a crucial document in order to save their beloved neighborhood at all costs.

Picking up where Hey Arnold! left off, this tells the story of what happened to Arnold's parents.