Costello's methodical modus operandi includes airtight alibis involving his lover, Jane (Nathalie Delon). Having carried out a murder contract on a nightclub owner, he is seen leaving the scene by several witnesses, including piano player Valérie (Cathy Rosier). Their statements are inconsistent but the investigating officer (François Périer) believes Costello is his man based on the witnesses who viewed Costello and his alibi that he was with Jane the whole time.