Drug dealer Šárlí ended up with a knife in his liver during a drunken brawl as a group of teenagers left a pub late at night. Commissioner Vašátko, who is already enjoying his retirement, is drawn into the investigation by the young man's grandmother, Forejtová, an old acquaintance of his late wife. It turns out that Vašátko's friend, the painter Horác, knows another member of the group, 16-year-old Zbyněk, whom he taught painting some time ago. Zbyněk is struggling with his authoritarian and precise father, the businessman Bouzek, and is clearly hiding something. Vašátko and Horác eventually uncover the background to the murder and its perpetrator. But first, they have to understand the titular "bug," as Vašátko calls the detail that doesn't fit in a criminal case. If the detail is not clarified, the case cannot be closed. In this story, the "bug" is a shoe found at the scene of the crime that does not belong to any of the participants in the altercation.