Commissario Guido Brunetti and Inspector Vianello play hooky to help get Vianello's friend Marco Ribetti--an environmental activist arrested during a protest against toxic waste being dumped into the city's waters--released from prison. But on the steps of the police headquarters, they come face-to-face with Ribetti's cantankerous father-in-law, who has been overheard making threats against Ribetti. And when the body of a night watchman is at the father-in-law's glass factory next to an annotated copy of Dante's Inferno, Brunetti must find out if there is a connection between this and whoever is ruining the waters of the lagoon.