1917, America joins the allied struggle in Europe by declaring war on Germany and humble factory worker Fred Williams is one of the first to volunteer to serve. Mary Phillips, the daughter of his boss, is quite proud of his bravery and patriotism.

Later in France they meet again, Fred awaiting deployment, and Mary participating in the entertainment division. Their love blossoms and they are married, but all too soon duty calls and Fred must go to the front, leaving behind his new wife and unborn child. Danger and complications mount; will Fred ever return to Mary or will he become one of the unknown fallen soldiers?

THE UNKNOWN SOLDIER starts out in life as Fred Williams (Charles Emmett Mack), a garage mechanic who signs up for the Army on the very day that America declares war against Germany. Upon arriving in France, Fred falls in love with wealthy heiress Mary Phillips (Marguerite de la Motte), who has come to the front as an entertainer. Opting for a quick marriage (after all, who knows what tomorrow will bring'), Fred and Mary allow a supposed chaplain to perform a ceremony -- but what they don't know is that the "chaplain" is a deserter with no authority to marry anybody. Mary finds this out on the morning after the wedding, but by now Fred has marched off with his regiment. Later on, Fred receives a letter indicating that Mary is about to give birth in a tiny French hospital which happens to be directly in the line of fire. He volunteers for a suicide mission in No Man's Land, hoping eventually to make his way to the hospital and rescue Mary -- at which point the film fades out. Fade in several months after the Armistice, with Mary and her child attending memorial ceremonies at the Tomb of the Unknown Soldier at Arlington Ceremony, tearfully wondering if the interred soldier might possibly be her beloved Fred.