The Girl (Lillian Gish) and the Boy (Robert Harron) are two young lovers who, despite both being American, both live in a small town in northeastern France. The presence of the Little Disturber (Dorothy Gish), a tomboyish sort with a hankering for the Boy, briefly threatens to disrupt their relationship, but the Girl and the Boy get past it and get engaged to be married.
Unfortunately the wedding of the Girl and the Boy is scheduled for early August, 1914. World War I intervenes, and the Boy volunteers to fight for his adopted homeland. The French make a brave stand but are soon forced into retreat, and the Germans take possession of the little village. The Girl's mother and the Boy's father are both killed by shelling. The Germans in the village conduct themselves like stereotypical rapacious Huns. One, a particularly loathesome officer named Von Strohm, lusts after the Girl.
Hearts of the World came about as a direct result of the British government hiring D.W. Griffith to make a pro-Allied film that would encourage American intervention in the war. By the time production started the United States had already declared war on Germany, but the film went ahead anyway, and it is a not-at-all subtle Propaganda Machine. The protagonists are Americans for no other reason than to show Americans volunteering to fight for France. Griffith, who was very anti-war, later regarded this film as an Old Shame.
- Almost Dead Guy: After the Girl stabs a German soldier In the Back and the Boy then shoots him, they think that he is safely dead. But he isn't quite dead yet, getting up and staggering down the stairs to the ground floor of the inn, thus alerting Von Strohm to the Boy's presence upstairs.
- Attempted Rape: Von Strohm is about to rape The Girl when he's called away by a superior officer.
- Battle in the Rain: The Boy infiltrates the German trenches and kills a couple of German officers during a heavy rainstorm.
- Bavarian Fire Drill: The Boy leans on this when he is skulking around behind enemy lines in a German officer's uniform. When a sentry challenges him, the Boy calls him out for smoking while on duty, and is allowed to pass undisturbed.
- Beta Couple: After the Little Disturber is rejected by the Boy, she latches on to Monsieur Cuckoo, a goofy-looking young man who had previously been admiring her.
- Blood from the Mouth: The Girl's mother has this after she staggers into a basement shelter, having taken a shell fragment in the guts. She dies soon thereafter.
- The Cameo: Griffith's prologue includes a shot of Griffith meeting PM David Lloyd George outside 10 Downing Street in London.
- Creator Cameo: The film starts with a brief prologue that shows Griffith in France shooting film in the trenches, then meeting Prime Minister Lloyd George in London.
- Foreshadowing: A German soldier decides to be a total creep and scare the Little Disturber by showing her how he could activate a grenade and throw it at her. At the end of the film, the Little Disturber saves the Boy and Girl by getting ahold of a grenade, activating it just like the soldier showed her, and chucking it at the German soldiers who are about to bust down a door and kill the Boy and Girl.
- Half the Man He Used to Be: One quick cutaway shot shows the Boy's father, lying in the street, neatly cut in half at the waist by a German shell.
- Heroic BSoD: "The mind of the simple soul broken by shell and terror." After the shelling and her mother's death, the Girl loses it for a while. She gathers up her wedding dress and staggers around town, thinking she will meet the Boy for their wedding.
- Historical Domain Character: Kaiser Wilhelm briefly pops up, plotting world conquest. Another scene has PM Herbert Asquith, Lloyd George, foreign minister Edward Grey, and Winston Churchill waiting out the final moments before expiration of the British ultimatum.
- Incorruptible Pure Pureness: Griffith's heroines were always pure and sweet, but this film takes it Up to Eleven when the Girl kneels at her bed and prays "Please make me so nice and good that Boy will love me forever and ever."
- Ironic Name: The Main Street in the sleepy little village is called "Rue de la Paix"—"Peace Street".
- Might Makes Right: "Might alone makes right", says Von Strohm the monstrous German officer when someone suggests that justice makes right.
- Neutral Female: Played straight and then averted. The Boy is engaged in a life-or-death grappling match with a German soldier. The Girl is off to the side cowering in true Neutral Female style, until she remembers that she has a big butcher knife, that she's been carrying ever since Von Strohm almost raped her. She then stabs the German soldier in the back.
- No Name Given: The Girl, the Boy, the Little Disturber.
- Scenery Gorn: Many shots showing the bombed-out ruins of what was once a lovely little French village.
- Street Performer: The Little Disturber is working as one in the village before the war starts. After the Germans sweep through and she's stuck behind enemy lines, she starts waitressing at the inn.
- A Taste of the Lash: One of the cartoonishly villainous Germans whips the poor Girl after she is too frail and week to lift a heavy bucket of potatoes into a cart.