Half-Scottish, half-Italian Enzo MacLeod used to be one of the top forensics experts in Scotland, and now he lives in Toulouse, working as a university professor. Divorced in Scotland and widowed in France, he has an estranged Scottish daughter and a French daughter he has raised by himself.

As if his life isn't complicated enough, he soon finds himself unexpectedly in the hunt for solutions to some vexing cold cases thanks to an ill-advised wager about the power of forensic science.

Meanwhile, in Paris, a man desperately seeking sanctuary flees into a church. The next day, his sudden disappearance will make him famous throughout France.

Deep in the catacombs below the City of Light, MacLeod unearths disturbing clues deliberately left behind by a killer. But as the retired forensics expert draws closer to the truth, he discovers he may just wind up the next victim for his troubles.