In a chapel on the estate of the "God’s Grape Harvesters", a Christian sect in Rosheim, reverend Samuel Toschi apparently died in the restoration of the frescoes on the vaulted ceiling. While the local police have already stopped the investigation, as they assume an accident, commissar Niemans, who has travelled from Paris, has come to the conclusion that this is a homicide case.
In his investigation, he is supported by the young police officer Camille, who has introduced to the sect as a seasonal worker for the grape harvest. As she finds in a cellar vault the vanished debris from the chapel, the suspicion that the death of Toschi is connected with the frescoes is confirmed. Niemans finds that there are precious frescoes from the Renaissance period among the paintings from the 19th century. While he and Camille are examining the ceiling vault with an x-ray apparatus, they discover another murder victim in the chapel.