The city has never had a sewage system; everything goes into the canals and then into the lagoon surrounding the city. The ecosystem can no longer cope with the amount of household chemicals being dumped; residents even had to leave the city one recent hot summer when the smell became unbearable.
But the real long-term danger, we discover, is the effluent from chemical plants on the nearby mainland. Some of the industries produce waste so toxic it even destroys the treatment plants installed to protect the lagoon.
We learn all this whilst following the work of Dr Giorgio Ferrari, the city's roving water-pollution inspector, a sort of David up against the chemical industry Goliaths.
It's easy to make the chemical industry look like the out-and-out villain: industrial plants look ugly - especially when intercut with classical Venetian architecture. And it's also easy to forget that we all need ballpoint pens and toothpaste. Chemical industries are vital to any economy. Unfortunately, tonight's film doesn't give much time for the industry to state its case.
As one inhabitant says, "Venice could easily become a dead city; a theme park for tourists." Responsibility for saving it lies with the young on both sides of the industrial debate. It's a dirty job, but someone has to do it.