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.
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 River Seine in Paris, along with murder weapons and other criminal evidence.
In the second programme Horatio gets a taste of life for the community of barge- and houseboat-dwellers who proliferate along the banks of the river.
He meets Jillie Faraday, an English woman who first came to live on her Dutch barge in the centre of Paris in 1969.
In four decades she's seen just about everything float past her home - from dead bodies to gigantic cargo barges which have come adrift from their tugs.
And whenever anything unusual or unsettling does come past, Jillie always phones the Brigade Fluviale.
Over the years she's got to know members of the Brigade quite well and has even asked their divers to retrieve keys and mobile phones accidentally dropped into the river from her barge.
Always looking for an opportunity to train, the Brigade are happy to oblige.
Horatio joins Jillie as her old friend, Chief Brigadier Pascal Jacquin, drops in on a routine call and they recall the incidents and accidents which are part of the flow of life on the river.
Horatio also takes part in a training session on board the Brigade's flagship, the Ile de France, a massive tug which can manoeuvre stricken cargo barges and retrieve sunken cars.
Horatio briefly finds himself driving the tug through central Paris and discovers that there's a considerable knack in not colliding with the city's famous bridges! And Pascal tells the story of navigating those bridges in the Ile de France while babysitting an unexploded Second World War bomb.
Horatio Clare explores life on the Seine with one of the barge-dwellers of central Paris.
In The Underwater Gendarme, writer and former lifeboatman Horatio Clare joins the Brigade Fluviale, an elite team which for over a century has been saving lives and recovering bodies from the River Seine in Paris, along with murder weapons and criminal evidence.
This week, Horatio is back out on an evening patrol with the Brigade.
As they work their way downriver through the heart of the city, Horatio marvels at the diversity of people who are irresistibly drawn to the banks and quays of the Seine - from musicians, dancers and lovers to dog walkers and drinkers.
And as the evening progresses, it's the drinkers who keep the Brigade busy, including one who prompts a call-out when he decides that a bridge parapet overhanging the river would be a good place to sleep!
Horatio also asks why the Seine which has attracted generations of writers.
He explores the river's role as a source of literary inspiration with the proprietor of a bookshop which has become a Parisian institution and with a journalist who lives on the Seine and has written the river's biography.
Horatio Clare discovers how the River Seine inspires lovers, writers and river policemen.
In The Underwater Gendarme, writer and former lifeboatman Horatio Clare meets the Brigade Fluviale, the Paris river police, an elite team which for over a century has been saving lives and recovering bodies from the River Seine, along with murder weapons and criminal evidence.
This week, Horatio joins the Brigade a few kilometres downriver from the centre of the city in a quiet backwater of Neuilly-sur-Seine.
They're here to investigate the disappearance of an elderly man from his houseboat and suspect that his body may be in the river nearby.
Today the Seine is calm and relatively warm but as the Brigade's diver searches the river bed, Horatio talks to the team about the difficulties of doing their job in strong currents, in waters close to freezing and zero visibility.
And one diver reveals that as well as discarded bicycles and cars, lost mobile phones and keys, the bottom of the Seine is also littered with miniature models of the Eiffel Tower!
Horatio Clare joins the Paris river police as they search the Seine for a drowned man.