From NetFlix:

A long-married Parisian couple (Charlotte Rampling and
Bruno Cremer) takes a vacation at the beach. But Rampling’s
life is suddenly thrown into turmoil when Cremer goes for a swim
and never returns. A penetrating character study into the nature
of denial, Under the Sand showcases Rampling at her very best in a
role that most comparable American actresses would kill for in
Hollywood.

The NetFlix review can be believed. This is a film for adults who want
to see adults act well. I highly recommend this film.