L'avventura (1960) DVD

A group of well-healed friends head out on a sailing trip to a isolated volcanic island in the Mediterranean. When time comes to return to the yacht, the group discover that Anna (the film's focus up to this point) is missing.
Sandro (Anna's flame) and Claudia (her close friend) try without success to find her.

While the rest of the group return to the mainland, Sandro and Claudia remain on the island to assist the Coast Guard in locating Anna. After some days with no sign of her, the pair decide to search back in Italy. During the time spent searching, Claudia and Sandro begin to become attracted to one another, and in due course they become lovers - all but forgetting about the still missing Anna.