In the early days of World War I, Karl Pfeiffer, a German-born American who made his wealth in the brewery business in New York City, stands in opposition to President Woodrow Wilson's decision to send American troops to Europe. He fears that the people in his "Fatherland" will be obliterated by the Allied forces. Because Karl is excessively proud of his German heritage and has his own peculiar notions about how best to bring about peace in the world, he becomes susceptible to the manipulations of saboteur Anton Miller. Miller introduces himself to Karl as "George Stewart" and convinces Karl to donate $50,000 to a supposed propaganda campaign designed to end the war. Karl tells Miller that he can pick up the check the following day at his Upper East Side apartment. That night, Maria, his wife of thirty years, and June Block, his soon-to-be daughter-in-law, are preparing a dinner for June's father, Henry Block. Henry is an assimilated German-American with political views that are in direct opposition to those held by Karl. As soon as the two men come together, their usual quarreling ensues. Karl's opinions and his explosive temper are familiar to all who know him, and it is because of this that his family has kept secret the fact that his son William has enlisted in the American army. The secret is soon out of the bag, though, when William arrives with news of his regiment's departure for Europe and requests that his family assemble for an early wedding. Karl is devastated by the news and angrily storms out of the apartment. He returns the next day and tries, unsuccessfully, to convince his son to reconsider his decision. When Miller collects the $50,000 check from Karl, he discovers that Karl is a friend of the wealthy Henry, and asks that he arrange a meeting with him. Soon after William boards his transport ship for Europe, Karl gets a telephone call from Miller, who informs him that his money was used to help sink the transport ship as it was leaving harbor. Now realizing that he was horribly misled by Miller, Karl vows revenge and plans to kill him. Henry's cool-headedness prevails, though, and the two decide to snare the saboteur instead. Soon after Miller arrives at Karl's to meet Henry, he is coaxed into revealing his true identity, at which point the police enter and arrest him. Karl's son, it turns out, did not perish in the ship tragedy and returns home safely. Reunited with his son, Karl vows to reject his earlier political beliefs and insists that henceforth his family refer to strudel as "apple pie." Karl then celebrates his newly found American patriotism by joining his family in a rousing chorus of "My Country 'Tis of Thee."