Péril en la demeure (1985)

The wealthy Tombsthays hire a guitar tutor, David Aurphet, for their daughter Vivian. The wife, Julia, begins a passionate affair with David. Edwige, Julia's new neighbour, also takes an interest in David. David is attacked one night on returning home but saved by a mysterious stranger Daniel who reveals himself to be a hired killer. David begins to suspect that his affair is no longer secret.