This is loosely based on Dostoevsky's White Nights. Quatre Nuits d'un rêveur doesn't have any obvious religious references as there are in some of his other films, although the male protagonist Jacques almost treats (mainly in private) Marthe (Isabelle Weingarten) as a god-like being. We first see Jacques hitch-hiking into the countryside to make somersaults in the grass, although in the evening he's back in Paris and sees what appears to be a suicidal young woman along the quai.
Jacques having prevented her from jumping, the two sit on steps and tell each other about themselves. Jacques is an artist in constant search for the right woman, whereas Marthe has already met the man of her dreams, the unnamed Jean-Maurice Monnoyer, whom she's known as a lodger in the flat she shares with her mother. He's had to leave for the States but promised he'll return a year from then, telling her where to meet him at a particular time exactly one year from then. But he didn't meet her.
Jacques escorts her home and they agree to meet, same time same place, the next night. This continues, and in spite of Jacques acting as a go-between, giving a letter written by Marthe to her lover's friends, he doesn't turn up. Meanwhile, in the day (between painting) Jacques is recording his love for Marthe on tape, the last recording just being a repeated 'Marthe' over and over. On the fourth night of meeting Marthe he confesses his love for her, she realises that her former lover didn't deserve her love, and they become lovers.
Er, well not physically, and spiritually only for a few minutes: Jacques buys her a scarf, they continue walking, Marthe sees her former lover, runs up and kisses him, goes back to Jacques and kisses him as a friend, and then returns to the unnamed lover. Time for Jacques to return home and back to his paintings the next day.