The film is a sentimental/ psychological drama centred on a couple, Jean-Paul (a taciturn Alain Delon) and Marianne (a radiant Romy Schneider), who are holidaying at a friend's villa near Saint-Tropez, in South-Eastern France. They have been together for a couple of years and seem to be very much in love. They are spending their time sunbathing, swimming (in the villa's swimming-pool) and having sex. Their bliss is interrupted, however, when Harry (Maurice Ronet) turns up with his 18-year-old daughter, Penelope (Jane Birkin). Harry is an old friend of Jean-Paul's. The simmering tension between the 2 men is at the heart of the story: it would appear that Harry was Marianne's lover before she met Jean-Paul. As for Penelope, she is not as passive and innocent as she may seem. The movie develops from there.
This is an excellent film. The story is plausible from start to finish. There is nothing dated about the plot or the actors' performance (the film was released in 1969). The psychology of the characters is captured in a manner that is both sharp and subtle. Inexorably, the tension between the characters rises. There is a question that you could ask yourself after seeing the film, and it will keep you thinking for a few days: To what extent are the various characters responsible (or not) for their actions, and for what unfolds? There is no simple answer.
An excellent film, close to being a masterpiece in my opinion, within the parameters of the genre.