A movie about the First World War based on a stage musical of the same name, portraying the "Game of War" and focusing mainly on the members of one family (last name Smith) who go off to war. Much of the action in the movie revolves around the words of the marching songs of the soldiers, and many scenes portray some of the more famous (and infamous) incidents of the war, including the assassination of Duke Ferdinand, the Christmas meeting between British and German soldiers in no-mans-land, and the wiping out by their own side of a force of Irish soldiers newly arrived at the front, after successfully capturing a ridge that had been contested for some time.

In the summer of 1914, leaders of European royalty gather together for a group photograph. As the camera's flashpan explodes, Archduke Franz Ferdinand and his wife fall dead. Accusations are exchanged, impossible ultimatums are delivered, and the heads of state choose sides for the inevitable war. A flashing electric sign at the seaside resort of Brighton announces World War I, and the Smith family falls in line to purchase tickets for patriotic sideshows from England's Field Marshal Sir Douglas Haig. Young Harry Smith responds to an enthusiastic music hall singer and enlists; a short time later, his brother Jack wins at a shooting gallery and is rewarded with a uniform and a one-way ticket to the war. Meanwhile, as Haig haggles with Sir John French and Sir Henry Wilson over strategy, a giant cricket scoreboard tallies deaths in battle. The social set, represented by Stephen and Eleanor, contributes to the effort by vowing not to drink German wine for the war's duration and by requiring their servants to knit mittens for the troops. On Christmas Eve, as the scoreboard tallies 1 1/2 million dead, Allied and German soldiers meet in the no-man's-land between their trenches to exchange liquor, cigarettes and addresses. While Bertram and George Smith live knee-deep in mud, the military brass drink champagne at lavish balls, and Haig mounts his tower at Brighton Pier to order 300,000 men into the Battle of Somme. Meanwhile, suffragette Sylvia Pankhurst is heckled by a streetcorner crowd as she reads anti-war letters written by George Bernard Shaw. At the front lines, a chaplain announces that the Dalai Lama in Tibet is offering his prayers, and the troops answer by singing their own bitter lyrics to traditional church hymns. At a field hospital, Harry Smith dies in the arms of his sister Betty, a nurse, while Haig cheerfully announces that, although no ground was taken at Somme, there were only 60,000 casualties. In 1917, the Americans enter the war, and victory is imminent. As a final tally shows nine million dead, military representatives meet on Brighton Pier to sign the peace treaty. On that day, Jack Smith is killed in action; following a maze of red tape, he walks through patches of swirling smoke to a path strewn with red poppies. Simultaneously, the Smith women picnic on the green hillside where the five Smith boys lie buried along with all the other millions of war dead.