Part-time model Valentine unexpectedly befriends a retired judge after she runs over his dog. At first, the grumpy man shows no concern about the dog, and Valentine decides to keep it. But the two form a bond when she returns to his house and catches him listening to his neighbors’ phone calls.

Set against Paris' oldest bridge, the Pont Neuf, while it was closed for repairs, this film is a love story between two young vagrants: Alex, a would be circus performer addicted to alcohol and sedatives and Michele, a painter driven to a life on the streets because of a failed relationship and an affliction which is slowly turning her blind.

Ninon is a spirited hotel-manager who teaches self-defense classes to her terrified eldery neighbors. Daiga, an aspiring Lithuanian actress newly arrived to Paris, becomes fascinated with the life of a mysteriously beautiful drag performer.

The image of a mysterious, solitary filmmaker - a cineaste maudit - who flees from both the media and the public, is unrelentingly bound to the figure of Leos Carax, in France. Elsewhere, the real focus is on his films and he is considered to be an icon of world cinema. Mr.X dives into the poetic and visionary world of an artist who was already a cult figure from his very first film. Punctuated by interviews and unseen footage, this documentary is most of all a fine-tuned exploration of the poetic and visionary world of Leos Carax, alias "Mr.X".