不幸的童年经历让十六岁的少年于贝尔（泽维尔·多兰 Xavier Dolan 饰）早早过上了同母亲（安妮·杜尔瓦勒 Anne Dorval 饰）相依为命的生活，然而，随着年龄的增长和叛逆期的到来，于贝尔渐渐发现，自己和母亲之间开始变得越来越难以沟通，两人之间的距离渐行渐远 。
内心充满了痛苦和矛盾，于贝尔开始了艰难的尝试，企图重塑他和母亲之间破裂的亲情与信任，可是，就在于贝尔的努力逐渐产生成效之时，他和同性男友安东尼（弗朗柯斯·阿诺德 François Arnaud 饰）之间的关系曝光了，这让他同母亲之间的关系再次回归了冰点之下。在安东尼的帮助下，于贝尔从寄宿学校出逃，偌大的天地之间，他能够寻找到一片得以藏身其中的小小天地吗？
Haunted by the memory of Walter Holderlin, a soldier he killed during World War I, French musician Paul Renard (Phillips Holmes) confesses to a priest (Frank Sheridan), who grants him absolution. Using the address on a letter he found on the dead man's body, Paul then travels to Germany to find his family. As anti-French sentiment continues to permeate Germany, Dr. Holderlin (Lionel Barrymore) initially refuses to welcome Paul into his home, but changes his mind when his son's fiancée Elsa identifies him as the man who has been leaving flowers on Walter's grave. Rather than reveal the real connection between them, Paul tells the Holderlin family he was a friend of their son, who attended the same musical conservatory he did. Although the hostile townspeople and local gossips disapprove, the Holderlins befriend Paul, who finds himself falling in love with Elsa (Nancy Carroll). When she shows Paul her former fiancé's bedroom, he becomes distraught and tells her the truth. She convinces him not to confess to Walter's parents, who have embraced him as their second son, and Paul agrees to forego easing his conscience and stays with his adopted family. Dr. Holderlin presents Walter's violin to Paul, who plays it while Elsa accompanies him on the piano. The film's original title, The Man I Killed, was changed to The Fifth Commandment to avoid giving wrong impressions in the minds of the public about the character of the story. It ultimately was released as Broken Lullaby.
