In Rome, former detective Franco Arno (Karl Malden), now a blind crossword puzzle maker, and his young niece Lori (Cinzia de Carolis) overhear a suspicious conversation while walking the street one night. Lori takes a look at the men. The next day, one of the men they heard turns up murdered, and Arno enlists the help of reporter Carlo (James Franciscus) to try to find the killer. There are other slayings, each of them involving a medical research institute, and then Lori is kidnapped while Carlo focuses his investigation -- and his romantic intentions -- on Anna (Catherine Spaak), the beautiful and mysterious daughter of a professor implicated in the deaths. It's up to the blind Arno to discover the clues that will save his niece from a madman with a disturbing motive.