Ambroise, a young doctor specializing in sleep disorders, spends the night with a patient suffering from violent sleepwalking. Isolated in his mountain chalet, he searches for answers to his nocturnal behavior. While reason dictates that she should leave as soon as possible, her obstinacy and fascination with the patient lead her to push the experiment further than she had planned.

When Nathan tries to motivate his friend Marceau to accompany him to the traditional July 14th ball, Marceau refuses. For the first time in years, Nathan and Marceau find themselves alone for the evening, their group of friends having left the country for the city. Marceau is finally persuaded, but they have to hurry, as the ball is a long way off...