It tells the story of a post-nuclear war experiment in time travel by using a series of filmed photographs developed as a photo-montage of varying pace with limited narration with only a single brief moving image.
Due to its brevity it often accompanies another film; Jean-Luc Godard's Alphaville (1965) was the film it was first released with.
Terry Gilliam's 1995 film Twelve Monkeys was inspired by and loosely based on La Jetée.
Warning: Wikipedia contains spoilers!
The plot is that the survivors of destroyed Paris live underground at Chaillot. They research time travel, hoping to send someone back to before World War III to recover food, medicine, or energy for the present, "to summon the past and future to the aid of the present". The traveller is a male prisoner, his vague but obsessive childhood memory of witnessing a woman during a violent incident on the main jetty at Orly Airport is used as the key to his journey back in time. He is thrown into the past again and again, repeatedly he meets and speaks to the woman who was present at Orly. After his successful passages to the past the experimenters attempt to project him into the future, in a brief meeting he is given a power unit sufficient to regenerate society. On his return to his present he is set aside to die but he is visited from the future and he asks to be returned to his childhood Orly. He is returned and finds the violent incident he partially witnessed as a child was his own death as an adult.