Niemans witnesses the death of a monk named Brother Guillaume who utters `the sword and the snake' with his last breath and has his palm burned to hide a tattoo. When Camille arrives to help investigate, they make the monastery their base.

It seems the monks believe they have discovered sinister secret messages in the ancient music they have been studying. As Camille and Niemans get closer to the truth, they encounter a satanic sect whose followers are seeking a blood sacrifice.

In a wine-growing region at harvest time, a battered corpse is discovered under rubble when a chapel vault collapses during the renovation of a chapel, destroying a fresco. As the case develops, it isn't long before another body comes to light.

When Niemans obtains X-ray images of the damaged frescos, they provide further insight into the case, but does a missing fragment from the biblical scenes portrayed hold the key to the killer?

In a blighted urban area, two men searching for drugs among the rubbish in an abandoned car park discover the severed hand of a child. When Niemans and Camille arrive, the police have not started investigating as they are distracted by another case.

Two more people are shot dead in the town's cemetery. Does a quotation from an ancient tale on a memorial provide a clue?