An American family takes in a young German relative and discovers he's been indoctrinated with Nazi propaganda.

The household of widower Mike Frame, a chemistry instructor at a Midwest university who is also engaged in secret war work, is anxiously awaiting the arrival of Emil Bruckner, the twelve-year-old son of Mike's late sister, who married and lived in Germany with the great German liberal leader Karl Bruckner. Both Bruckner and his wife perished in a German concentration camp, and consequently, Emil, now orphaned, is being sent to live with his American uncle. Mike's daughter Pat eagerly anticipates the arrival of her cousin, while Mike's spinster sister Jessie, an ardent anti-Nazi, disapproves of the boy joining their household. Frieda, their German housekeeper, is happy to welcome her fellow countryman, and Leona Richards, a Jewish schoolteacher and Mike's sweetheart, is warmly supportive of Mike's decision to accept him. When Emil fails to arrive on the train, Pat and Frieda become worried. Having just proposed to Leona, Mike invites her home for lunch, and soon after they arrive, Emil appears and calmly announces that he has come by plane. The extremely formal boy is suspicious of his new family's friendliness and voices a loathing for his father, who has once Mike's teacher. Thunderstruck upon discovering that Leona is Jewish, Emil makes an antisemitic remark and then goes to his room. After he leaves, Mike and Leona discuss getting married, even though their marriage would mean that Leona must refuse a lucrative offer from a Chicago school. In his room, Emil changes into his Hitler Youth uniform and later savagely attacks Frieda when she rejects his attempt to recruit her to the Nazi cause. Terrified by the boy's aggressiveness, Frieda calls for help, causing Leona and Mike to have a serious talk with the boy. After answering their questions with Nazi stock answers, Emil accuses his father of being a traitor because the liberals caused Germany to lose the war in 1918. After Mike and Leona leave the room, Emil viciously slashes the portrait of his father that is hanging on the wall. At school, Emil is assigned to Pat's class, which is being taught by Leona. With his arrogance and intolerance, Emil quickly engenders the hate of his classmates. Upon learning of Mike's impending marriage, Emil launches a campaign to divide the family and manipulates Jessie into feeling that Leona is breaking up her home. When Emil gets into a fight with Stan, a boy of Polish descent, Millie, a classmate whose American father is being held prisoner in a German camp, testifies that Stan attacked Emil. Later, Millie confesses that Emil threatened to write to Germany and have her father executed unless she testified for him. Forced to apologize before the class, Emil screams that he is being persecuted and, after running out of the classroom, he uses a piece of chalk to scrawl on the sidewalk that Leona is a "Jewish tramp." Emil then runs home and shrewdly apologizes to Mike. When Leona demands that Emil be punished for his actions, Mike, who believes Emil's insincere apology, defends the boy, causing Leona to break their engagement and accept the job in Chicago. Soon after, Pat catches Emil rifling Mike's desk in search of secret papers from the War Department, and when she threatens to tell Mike, Emil attacks her with a poker. When Pat is discovered unconscious, Emil flees and Mike sends the police after him. The incident brings Leona back to the house and she reconciles with Mike and Jessie. Stan and several of Pat's friends chase Emil, and after Stan thrashes him, the boys take him back to the house. Insane with rage, Mike almost chokes the boy to death before Leona stops him. After sending Mike out of the room, Leona talks to the boy and finds him subtly changed. When Emil discovers that Pat has borrowed a year's advance on her allowance to buy him a watch for his birthday, he breaks down and cries. Mike calls the police to take Emil away, but Pat, now conscious, and Leona urge him to give the boy another chance. When Emil stammers that he now realizes that his father was a brave man and recalls the beatings he suffered at the hands of the Nazis, Mike understands that the boy has been an innocent victim of the Nazis and allows him to stay.