Bert is finally turning 13. After years of waiting, he is finally becoming a teenager. For his birthday, he wants Rebecka from the parallel class, wrapped in shiny foil or jumping out of a cake. The worst thing that can happen in life is a health check, especially if you are 13 years old. Bert, Åke, Klimpen, Torleif, and the other boys in the class sit petrified with fear, waiting for the extremely terrifying Sister Ilse. The class is raising money for a class trip. Bert comes up with an idea—they will organize a gala where Bert's rock band, Heman Hunters, will play. At the same time, Klimpen threatens to perform as a stand-up comedian. Bert's new love is called Nadja Nilsson. Through Torleif, he finds out that she plays the violin, which Bert immediately starts practicing instead of soccer. But what will he become? A violin-playing phantom in love or a celebrated goal scorer?

During a school trip with his parallel class, Bert plans to make a move on Paulina. At the same time, the bus gets stuck in the middle of the forest. Lill-Erik seizes the opportunity to take a pee, only to be left behind by the bus. The sun is shining and it's summer vacation. Bert wants to go to the beach and check out the girls in bikinis in general, and Paulina in particular. Bert's dad is going through a midlife crisis. Bert is forced to take over his dad's optician shop because his dad Fredrik would rather play with Bart's rock band "Heman Hunters". This turns out to have certain advantages, because a wonderful, fantastic girl comes to the shop. It's time to start middle school. Åke gets off to a strong start by blowing up the chemistry lab, and Bert falls in love with Ida. But Ida is with a guy called "The Gravedigger." For some reason, he wants to skin Bert alive.

The film is based on a series of immensely popular Swedish children's books, about the boy Bert, who is just hitting puberty and having the usual problems with it. But that's where the usual ends abruptly.