Narciso Ibáñez Menta is the widowed father of a son. Although a mathematician, since the death of his wife, he has made his living by writing pulpy mysteries under a pseudonym. When the boy is killed in a traffic accident, he plots the driver's death.... and the driver dies by poison. Menta has a perfect alibi.
The first adaptation of Cecil Day Lewis' novel shows some interesting parallels to Day Lewis' life, although he was not, so far as I can tell, implicated in a murder. The distinguished poet, who became the Laureate in 1968 wrote mysteries under the pseudonym of 'Nicholas Blake. The movie is a clear film noir, with lots of foggy night scenes and a major flashback, visually interesting and with a nice degree of sexual heat running through it, that Chabrol's later version muted.