Jonathan (HAUKE DIEKAMP) has already been kicked out of eight boarding schools. When he starts at the boarding school of the famous Thomanerchor in Leipzig, he doesn't think he will stay there for long. But Justus (ULRICH NOETHEN), the head of the boarding school, takes care of him. And indeed, Jonathan quickly makes friends. The boys from his dormitory become his friends: the level-headed Martin (PHILIPP PETERS-ARNOLDS) and the lanky Uli (HANS BROICH WUTTKE) as well as the experimental Kreuzkamm junior (FRANCOIS GÖSKE) and the always hungry strongman Matz (FREDERICK LAU). The four show Jonathan their secret meeting place, an old railroad car on a vacant lot. That same evening, they are surprised there by a mysterious man, the Non-Smoker (SEBASTIAN KOCH), who also has a key to their railroad car. The next morning, a concert recording of the boys' choir is scheduled in the Nikolaikirche. But there is a clash with the "externs," the classmates from Leipzig who do not live at the boarding school. The Thomaner have been at loggerheads with them for ages. The externals steal the concert notes, kidnap Kreutzkamm and the concert becomes a fiasco. To polish up their reputation, Justus orders the boys to put on an excellent Christmas performance. But finding a good play for it proves to be very difficult. Then the children discover a yellowed folder with a theater text in the train car. The title is "The Flying Classroom. Wow, what a text! The children are on fire and want to put on a kind of space opera à la Starship Enterprise. Jonathan composes the music and directs the play. And the externals, with their leader Mona (TERESA VILSMAIER), are also supposed to play their part. But everything turns out differently. When Justus overhears a rehearsal, he summarily forbids the performance. Jonathan is furious and flips out. He destroys the scenery and, by an unfortunate coincidence, starts a fire: the boarding school is on fire! The fire department has to come. Jonathan finally admits his guilt to the director, but Justus also admits that he is partly to blame. He should not have banned the performance without comment. But the play reminded him of his own difficult boarding school days. His best friend at the time, Robert, whom he has never seen again since his escape from the GDR, had written it. At the end, the children have a surprise for their leader: they reunite the two old boarding school friends. Because - as it turns out - the non-smoker is none other than Justus' old friend Robert. Last but not least, there is also an impromptu Christmas performance in which the Thomaner sing together with the externs. And Jonathan is looking forward to staying a long time at this boarding school, where he has finally made friends.