The novel is a work of historical fiction set during the Winter War (November 30, 1939, to March 13, 1940), a 105-day conflict in which Stalin's Soviet Union invaded Finland, a small nation that had gained independence from Russia only 22 years earlier. At the center of the story is Simo Häyhä, a Finnish farmer and marksman from the village of Rautjärvi near the Russian border, who becomes the deadliest sniper of the war and earns the nickname "the White Death."
The novel opens with a prologue surveying a battlefield littered with the dead of both armies, their blood soaking into Finnish soil. A second prologue frames the conflict: Finland, wedged between Nazi Germany and the Soviet Union, faces Stalin's demands for territory and military bases, not because Stalin fears Finland itself but because he fears Hitler could march through Scandinavia to attack Leningrad.
Before the war, Simo is a quiet young man who reads the forest instinctively, tracking animals and judging distances with skills his father taught him. While hunting, he spares a fox and a pregnant vixen, drawing a distinction between being able to kill and having to kill. At a championship organized by Finland's Civic Guard, a voluntary militia, Simo hits the bull's eye 16 times in one minute with a manual-loading rifle, outperforming a career officer using an automatic weapon. His friends Onni, an optimist whose wedding is approaching, and Toivo, his closest companion, celebrate with him. On the ride home, Toivo relays troubling news: Officers' leave has been cancelled, and reservists will reinforce the border.
As tensions mount, Field Marshal Carl Gustaf Mannerheim, the elderly head of Finnish defense, urges his government to yield to some of Stalin's demands, but his pleas go unheard. In Moscow, Stalin's government engineers a pretext for war: A squad recruited from a gulag, a Soviet forced-labor camp, shells a Russian garrison at Mainila from Russian territory, killing Soviet soldiers, and Russia blames Finland. Molotov, Stalin's foreign affairs chief, renounces the non-aggression pact.
Finland mobilizes. Pietari Koskinen crushes his younger brother Viktor's fingers with a sledgehammer to spare him from service. Leena Aalto, a young Lotta (a member of the Finnish women's auxiliary organization), receives her papers eagerly. Some 300,000 men and 100,000 women leave their homes. Simo gives his first rifle to Onni, since many reservists lack weapons.
Simo, Toivo, Onni, and Pietari are posted to the 6th Company of the 34th Regiment at Kollaa, a remote front of forests and marshes near the Russian border. Their commander is Lieutenant Aarne Juutilainen, a former French Foreign Legionnaire known as "the Terror of Morocco"; Karlsson, a platoon commander, is the only officer who shows concern for the men. Hugo, a gentle giant, joins Simo's circle of friends. Over 45 days of digging trenches and forced marches, morale deteriorates as no invasion materializes.
On November 30, the Soviet Union invades. Russian bombers devastate Helsinki, and attacks unfold along the entire border. At Kollaa, the 6th Company burns border villages to leave nothing for the advancing enemy. Juutilainen disobeys retreat orders and leads the company to ambush a Russian column, where Simo kills his first men: five mounted officers in five seconds. The Finns discover the Russians wearing summer clothing, poorly equipped for Arctic forest warfare.
The 6th Company earns the nickname "war thieves" through raids on Russian supply columns, stealing weapons to supplement Finland's scarce arsenal. Simo refines his techniques: smearing ashes on his rifle barrel to prevent glare, placing snowballs in his mouth to mask his breath, and never using a telescopic sight. With Toivo as his spotter, Simo becomes devastating. The Russians begin whispering about "Belaya Smert," the White Death. On the Mannerheim Line, Finland's primary fortified defense, Viktor enlists despite his crushed hand, driven by shame, and survives the destruction of his company on the war's first day. International sympathy pours in, but no country sends troops.
The Soviet high command dispatches Lev Mekhlis to the front. Finnish forces ambush his disastrous frontal assault along the Loimola road; Mekhlis barely survives and executes the officer Borodin and the political commissar Sadovski to deflect blame.
Toivo is killed by a Russian sniper. Simo wraps his friend's body, snaps his dog tag, and prays beside him until he nearly freezes. He then hunts the sniper, firing from 490 meters without a scope, a shot never matched or explained. Rather than finding peace, Simo plunges into bloodlust, killing 24 Russians on Christmas Eve. Onni says he no longer recognizes his friend.
Stalin appoints Simeon Timoshenko to restructure the Red Army with better clothing, ski training, flame-throwers, and limitless munitions. Through January and February, a war of attrition grinds on at temperatures reaching minus 51 degrees Celsius. Juutilainen's display of impaled Russian corpses around his tent jolts Simo from his violent spiral; recognizing in the legionnaire's savagery the abyss he is approaching, Simo returns to his friends.
Simo takes the young recruit Yrjö under his wing after noticing the boy wears Toivo's mended uniform, identifiable by the bullet hole and a square of Finnish-blue cloth Leena had sewn into the collar. Simo transmits all his sniper knowledge in a single extended lesson. When 20 Russians corner them near a mass grave, Yrjö saves them by shouting that he is Simo Häyhä, the White Death. The Russians flee rather than face the legend.
France's Prime Minister Daladier promises Finland troops, encouraging the government to reject Stalin's peace terms on March 2. No Allied soldiers ever arrive, prolonging the war by two weeks. In the fighting, Pietari is engulfed by flame from a Russian tank; Simo mercifully shoots him to end his agony. At the battle of Ullisma, where 2,000 Finns attack 12,000 Russians south of Kollaa, Karlsson is fatally shot.
On March 6, an explosive bullet fired by a Russian sniper, a type of munitions that may violate the 1868 Saint Petersburg Declaration, passes through Simo's cheek and detonates against his jaw, tearing off both cheeks and part of his jaw. Leena discovers Simo among the dead at the bivouac and twice saves his life during the evacuation, including clearing debris from his blocked airway. Surgeon Aarne Ellonen performs an emergency tracheotomy without anesthesia and reconstructs Simo's jaw with metal wires.
By March 8, all fronts near collapse. Mannerheim tells his trusted assistant Aksel Airo they must negotiate before Stalin realizes one more push could take all of Finland. On March 12, Finland signs the Moscow Peace Treaty, ceding 10 percent of its territory and the homes of nearly half a million citizens. Stalin secretly orders the Red Army to expend all remaining ammunition before the ceasefire at 11:00 on March 13. Viktor survives the final barrage and drops his rifle forever. At Kollaa, Yrjö is killed in the war's final minutes, his fate mirroring the opening prologue.
Simo awakens from an eight-day coma on March 13. Through written notes, since he cannot speak, he learns that Kollaa held but that his farm now lies on the Russian side of the redrawn border. He has lost his face, his home, Toivo, Pietari, Hugo (who also perished in the fighting), Karlsson, Yrjö, and the right to speak of his deeds, since his identity must remain hidden from Russian spies. The epilogue notes that nearly 400,000 Soviet soldiers were killed, wounded, or went missing, while Finland lost close to 70,000. Russia's poor performance emboldened Hitler to launch Operation Barbarossa, the largest invasion in military history, suggesting the Finnish resistance altered the trajectory of the Second World War.
A Second Epilogue, set in 1976, finds an elderly Simo living alone on a government-provided farm, never having married. When a journalist visits, Simo deflects praise, saying he did only what was asked. President Urho Kekkonen arrives, and the two old friends take their rifles into the forest. "We'll see what the forest has to offer us, and see if we should take it," Simo says (332), echoing the distinction between being able to kill and having to kill that has defined his life.