Arnold and the gang must stop Big Bob from chopping down the oldest tree in town.
The kids are proud of their tree house built in the branches of Mighty Pete, the oldest (and only) tree in the neighborhood. They are confident that the tree will be there forever — until Big Bob announces that Mighty Pete will cut down and build a beeper center on its place. Friday the 16th is the day.
Arnold wants to fight for the rescue of the tree but his friends don't believe that kids could do anything about it. To help Arnold, Helga tries to persuade her father not to chop the tree down without success. Reluctantly she decides to be on his side this time. When Arnold tells his grandparents about Bob's plan, Grandma is immediately willing to help him. The next day they visit the city hall together but there they don't get any help. So they put posters on the walls and hand out fliers. Subsequently Grandma (in her hippie costume) and a few kids march in front of Big Bob's house. But he doesn't really care.
The night before Friday, Arnold can't sleep and decides to go for a walk to see Mighty Pete one last time. There he unexpectedly encounters his classmates who couldn't sleep either. Together they climb into the tree house. Soon Bob and Nick Vermicelli arrive to bulldoze the tree at exactly 12:01 AM on Friday the 16th. Arnold explains to them that the kids are not coming down. To Arnold's surprise even Helga joins them. Nick is not impressed at all and steers the bulldozer in the direction of the tree to scare the kids and make them leave the tree. Bob wants to stop Nick because he doesn't want Helga to get hurt and starts to punch him. In the heat of the moment both of them fall off the bulldozer which continues to move towards the tree with no driver. The kids start throwing all the furniture onto it to stop it but without success. Just when they are going to give in and flee, suddenly Grandma appears in a Tarzan costume. She swings from a liana, lands on the bulldozer and manages to stop it in the nick of time.
So Bob decides not to cut down the tree instead he plans to build his beeper center on Gerald Field. Arnold and the gang then run after Bob and tell him that he can't do that.