In 2019, his mother Regina owns and operates the Waldhotel Winden, and his father Aleksander is the director of the Winden Nuclear Power Plant. This places Bartosz among the wealthy elite in a town of mostly blue-collar power plant employees, but also isolates him from his classmates. Jonas is his only friend, and when Jonas is not around Bartosz spends his time alone smoking pot and playing video games. He seeks ways to prove himself and to fit in.
When Jonas was in treatment after his father Michael's suicide, Bartosz told their classmates he was on an exchange program in France, trying to protect him. At the same time, he became romantically involved with Martha Nielsen, who had been Jonas' girlfriend over the summer.
After the disappearance of Erik Obendorf, Bartosz leads Jonas and the Nielsen siblings into the woods near the Winden Caves in the hopes of finding Erik's stash of drugs. When they arrive, Franziska Doppler has already 'claimed it'. Bartosz takes it from her forcefully by pushing her to the ground. The group is frightened by loud roaring coming from the cave and scatters. A few days later, he receives a call from Noah, and thinking him to be Erik's supplier, plans to meet him with Jonas later. Bartosz goes alone, however, when Jonas never shows up.
On the afternoon of November 11, Bartosz is surprised to meet his grandmother, Claudia Tiedemann, whom he had thought was dead. He meets with Noah that night and, as events have transpired as predicted since their first meeting, agrees to do as he says. The following night, he meets with Noah again and the priest explains the nature of their war. Bartosz learns from Noah that his grandmother Claudia is of the shadow and that she is their primary enemy, that Jonas Kahnwald is merely her puppet in this war, and that they belong to the light although some of what they do may be dark. Noah then gives Bartosz his triquetra notebook.
It is also possible that Bartosz is a descendant of Tronte Nielsen due to the later having an affair with Bartosz's grandmother Claudia.