The film Once Upon a Time is based on the short story, Pour une nuit d'amour written by the French novelist Emile Zola (1840- 1904)

The young ravishing girl descending from a wealthy and noble family, whose strong character and the strange behaviour make her parents unhappy. The parents decide to send her abroad to do her studies and also intending that their daughter will be well disciplined.

The daughter and the nanny's son, who becomes a lawyer later on with the help of the noble family, had a strong friendship during their childhood. The parents are trying to find a suitable partner to their young daughter who has come back after completing her studies.
  
The daughter and nanny's son falling in love. The nanny's encouragement and the blessings strengthen the young couple's love. The young girl falls in love with the music played by the ugly, bulky orphan with a deformed face who is living in front of this rich family. Her passionate to the music will lead to a magnificently melodramatic climax as this deformed person imagines that he loves this young girl loves him, and his entire existence builts around young girl's love.

As a result of this misunderstanding, the young couple confront with many difficulties. But one day the young girl beckons the deformed person to help her to dispose her lover's dead body. The inexorable fate of the two lovers dreaming of owning this beautiful girl will be the same?