Arnold and his classmates try to avoid getting thrown in dumpsters and trash cans as part of an annual ritual.
During lunch at P.S 118, everyone is nervous because it's the first Monday in June, otherwise known as "Trash Can Day." Eugene asks what Trash Can Day is and Sid and Gerald explain that it's a tradition where 5th graders all over the city put 4th graders into trash cans. It's always on the first Monday in June because the trash is picked up the next day, so there are plenty of cans outside. Arnold tells everyone they will be fine as long as they stick together. Meanwhile in another part of the school, Wolfgang, a burly 5th grader, prepares a gathering of his 5th grader friends to trash can the 4th graders. He tells them the most efficient way is to pick them up under the armpits and drop them in the can feet first. Edmund, Wolfgang's sidekick, initially does it wrong when doing an example with a dummy they are using but is instructed how to do it right by Wolfgang. Arnold prepares a group of his 4th grader friends, consisting of Gerald, Sid, Stinky, Helga, Harold, Phoebe, Iggy, Curly, Eugene, and Joey, to avoid being trashed by organizing a path to go to the flower shop, which is eight blocks away, and then to his house. Eugene tells them that Park has a safehouse around the local junkyard. Arnold says it's a good idea that they can wait there until the coast is clear since it's halfway to the flower shop.
At three o'clock, when Trash Day is supposed to start, everything seems clear. That is, until the 5th graders corner Arnold's class in the hallway and two of them each bring a trash can. Harold panics and yells for everyone to fend for themselves, causing chaos. Some kids aren't lucky and are put into trash cans, including Curly, Joey and Iggy. Phoebe and Stinky avoid capture by hiding: Phoebe hides in a locker and Stinky dives into an empty trash can and puts the lid on (to which he mutters "Talk about ironic!"). A few 4th graders are able to escape out the front door and run off. Arnold, Gerald, Sid and Eugene run into the basement. They climb onto a table to look out the window and watch plenty of 4th graders being dumped into trash cans and dumpsters. Harold sprints down with a pair of 5th graders chasing him and bumps into the table which causes it to shake. Arnold, Gerald, Harold and Eugene manage to escape through the door, but Sid jumps onto a generator and is caught from behind by a 5th grader. Arnold and Gerald watch helplessly as Sid is picked up kicking and screaming before being dropped into a trash can. The four of them run out of the playground and down an alley toward a high fence they can't climb. Arnold sees a hole in the bottom of the fence and crawls through it as Wolfgang and Edmund appear. Gerald crawls through next followed by Harold, who pushes Eugene out of his way to escape first. Eugene nearly makes it, but Wolfgang grabs his ankle and starts to drag him back. Gerald grabs Eugene's arms and tries to pull him free, but loses his grip. Wolfgang throws Eugene into a trash can and makes the hole in the fence bigger so he can get through. Arnold, Gerald and Harold get into a bus to get away, but discover that three 5th graders are on it. Arnold stops the bus and he and Gerald escape, but the door closes on Harold and he's caught.
Arnold and Gerald are chased by Wolfgang and Edmund for a few blocks. As the two 5th graders round a corner, they see their prey is gone and keep walking. As it turns out, Arnold and Gerald hid in the sewer underneath them. They crawl out of a sewer pipe into an empty junkyard where Park's safehouse is located. They both go inside and see every kid who was put into a trash can thus far and other kids just hiding there for safety. A few kids hose down Eugene, and Harold tells Helga what happened to him after he was caught in a mock-therapy session. Everything is calm until Wolfgang breaks the safehouse door, leaving Park and the others wondering how they found it. Wolfgang throws a trash can in and the rest of 5th graders come down. Arnold, Gerald, Helga and Sid manage to escape, but apparently some of the 4th graders in the safehouse are put into trash cans again.
Helga and Sid run off in another direction while Gerald and Arnold wonder what they can do because the streets are filled with 5th graders. The two are greeted by the mysterious Mickey the Weasel, who has agreed to help them at a cost. Arnold gives him a yo-yo and dog toy and he accepts them. Mickey leads Arnold and Gerald through the streets and they constantly hide from Wolfgang, Edmund and the other 5th graders. Mickey finds a wooden dolly in one of the buildings and has Arnold and Gerald lie on it with a sheet covering them. Mickey wheels them to another location and when the sheet is taken off, Arnold and Gerald find that he's led them straight to Wolfgang and the 5th graders! Arnold yells at him for tricking them and Mickey, who is a 5th grader himself, replies with a laugh, "They don't call me the Weasel for nothing!". Wolfgang picks up Arnold and Gerald to put them in a trash can. Arnold tries to reason with Wolfgang, saying he'll just be continuing the vicious cycle that's been there for generations and adds that he'll be seen as a hero if he stops Trash Can Day. Though Wolfgang considers this, he throws Arnold and Gerald into a trash can anyway, then tips it over and rolls it down the hill. As Arnold and Gerald climb out covered in garbage, they agree that it wasn't as bad as they thought it would be. Three 3rd graders nearby laugh at them, but Gerald says it will be their turn next year. The 3rd graders fearfully ask if they'll do it, but Arnold says they won't. However, he thinks they should still keep up the tradition next year. They leave to go get cleaned up and Stinky is shown hiding in another empty trash can.