The Brigade Fluviale, the Paris River Police, know the Seine better than anybody else.
They understand its moods, the dangerous currents which can suddenly pin a diver to the bottom or sweep them far downstream.
They know the places where stolen cars are dumped and where the river gathers the bodies of the drowned.
In The Underwater Gendarme, writer and former lifeboatman Horatio Clare joins the Brigade Fluviale, an elite team which for over a century has been recovering the drowning and the drowned from the Seine, along with murder weapons and other criminal evidence.
In the first programme Horatio meets Pascal Jacquin, an expert diver and boat handler who has worked for the Fluviale for 27 years.
Three times a week Pascal takes an early morning swim around the Ile de la Cité, the island heart of Paris.
This three-kilometre dip is essential not only for fitness but also to give Pascal a daily sense of the 'mood' of the river.
He compares the river to a lover.
At any moment he could find himself called out to dive for a body or a piece of evidence and he needs to be prepared.
Horatio also meets Eric Calvet who leads Squad A3 of the Brigade Fluviale and joins him on an evening patrol.
Suddenly Horatio finds himself involved in an emergency call-out when they're deployed to rescue a young woman who's reported to have thrown herself into the Seine from the Pont de la Concorde.
These programmes are not just about the Brigade.
They're also a portrait of the river itself.
We see the Seine through the eyes of people whose lives are intimately bound up with it.
In the first programme the theme is love - from the unhappy affairs which have prompted so many lovers to jump from the city's bridges to Pascal's own love affair with the river which flows through the heart of Paris.