In a Munich Masonic lodge, the initiation ritual for the new member Ursula Falk is marred by a tragic event: chairman Georg Engelmann is struck by a falling chandelier and dies. It soon becomes clear that it was not a tragic accident but a planned murder. Marie Blank begins to investigate, but runs into a wall of silence among the members of the lodge. Also interested in the case is Father Castell, who comes to her aid. The victim, Georg Engelmann, was an architect and had stolen a "keystone" from the vault of an old church he was working on. He then ordered a replacement stone from the stone cutter Josef Grantziger. What is the meaning of the stone, and does it conceal the motive for a murder?
While Marie is investigating Engelmann's private sphere, and closely scrutinizing his son Peter as well as Ursula Falk, the victim's girlfriend, Father Castell goes undercover in the Masonic lodge in order to obtain information from Gerome Fabio and the other Masons, and break through the wall of silence.