Franco, a blind man that loves solving puzzles, and Carlo, a journalist, get caught up in a mysterious series of murders and break-ins at a local research facility, where no one seems eager to discover the truth.
Dario Argento has stated that of all his films, this was his least favorite
One night, while walking home with his niece, Franco Arno, a blind ex-newspaper man, overhears a conversation between two men in a car parked outside of the Terzi Institute. The next morning, it is discovered that there was a break-in at the institute. Police and reporters show up, but the institute directors deny that anything was actually taken. One of the reporters, Carlo Giordani, is especially curious about the case.
Later, a scientist at the institute, Calabresi, tells his fiance, Bianca Merusi, that he has discovered what was taken during the break-in. Shortly thereafter, Calabresi winds up dead, "accidentally" falling in front of a train at the station. When a picture reveals that Calabresi was actually pushed in front of the train, the photographer is murdered before Giordani and Arno can arrive and the negative has vanished.
As Giordani and Arno delve deeper into the mystery, they discover there are nine leads in the case: five scientists, the director's daughter, Bianca Merusi, the missing photographs, and the break-in at the institute. As Giordani and Arno track down each of these leads and eventually discover what type of research is actually being conducted at the institute, the killer tries to stay one step ahead of them.