Nurse Laura Mattson Diana Wynyard and World War I military pilot Lt. Geoffrey Aiken Robert Young fall in love after only knowing each other for a few days. Geoffrey is fatally wounded on his very first mission and brought to her hospital; after he dies, Laura discovers she is pregnant. Edward Seward Lewis Stone loves Laura and persuades her to marry him, and, as far as anyone knows, the child will be his.
By 1940, Laura's son Bob Seward Phillips Holmes has grown into a young man, newly engaged to Peggy Chase Ruth Selwyn. Laura has raised Bob to embrace pacifism. Meanwhile, Edward Seward, now United States Secretary of State, flies home after having negotiated the Seward Peace Treaty, which he claims will make it impossible for any country to go to war again. However, when the U.S. ambassador to Eurasia is assassinated while en route to the Eurasian State Department to discuss an earlier diplomatic incident, the President of the United States sends the Navy across the Atlantic Ocean to underscore the U.S. demand for a formal apology. Eurasia refuses to comply, and another world war becomes inevitable despite the treaty.
Laura speaks at a large peace rally, over her husband's strong objection. The rally is broken up by a group of angry men. A mob then gathers at the Seward home and starts pelting the place. Edward manages to disperse the crowd by first reminding the mob of each American's right to voice his or her own opinion in peacetime, and pledging himself wholeheartedly to the struggle once war is declared. When a news reporter interviews him, he insists his son will enlist. Bob categorically denies this, causing Peggy to break off their engagement. Unable to get his son to change his mind, Edward tells him that he at least has no right to sully the Seward name, revealing that he is not Bob's father. Laura confirms it, and tells Bob of his real father and how he died.
War breaks out. Privately, Edward informs his wife that the war is going badly because America fell behind during the years of peace; the “Canal” has been captured by the enemy, and 12,000 U.S. troops killed in two days by enemy gas bombs. When Eurasia launches an air raid on New York City, destroying such landmarks as the Empire State Building and the Brooklyn Bridge, Laura is injured, though not seriously. Bob changes his stance and enlists, not in the chemical division as Edward had suggested, but as an aviator like his real father. Bob and Peggy marry, then he departs with his squadron. As she watches Bob's squadron fly over the city, Laura now understands that freedom is not free; that we must always be prepared to safeguard it; and we all have a responsibility to defend it.
