Charlie the main character is a tortured character. At first we see him as just a simple lounge like piano player, and then his brother shows up. His brother knowing that he's being followed, decides to go to Charlie for help. Unfortunately by him doing this sends the thugs that he was running from to start to go after Charlie, and anyone who is close to him.
Charlie is constantly torn throughout the film by his shyness, and can't seem to act on his impulses. There is a flashback of Charlie when he was Edouardo, which gives the explanation of how Charlie has gotten where he's at in life. This I think was the most interesting part of the movie, it helped me get a closer, more personal look into how Charlie ticks. We see that his pianist success was helped along with his cheating wife, who in the end couldn't forgive herself for what she had done to him, and the marriage. His wife begs for him to leave because she's disgusted with herself, he leaves but in his mind he knows that she wants him to stay, and deep down he wants to stay to. Unfortunately his hesitation, in the end has dire consequences, his wife plunges herself out the window to her death.
After taking this flashback the viewer can be a little less frustrated with Charlie and his inability to act on impulses, and to be timid.
It's interesting to me that Truffaut made 400 Blows before this film, this filmed did not seem to be as refined as 400 Blows.