A fictional account based closely on Linna's own experiences during the war, the novel presented a more realistic outlook on the formerly romanticized image of a noble and obedient Finnish soldier.

Although published to mixed reviews, The Unknown Soldier quickly became one of the best-selling books in Finland and is considered both a classic in Finnish literature and a part of the national legacy.

The novel follows soldiers of a Finnish Army machine gun company operating on the Karelian front during the Continuation War from mobilisation in 1941 to the Moscow Armistice in 1944.

The novel paints realistic, yet sympathetic, portraits of a score of very different men: cowards and heroes—the initially naive and eventually brave upper-class idealist Kariluoto; the down-to-earth Koskela; the hardened and cynical working-class grunt Lehto; the company comedian Vanhala; the pragmatic and strong-nerved Antero "Antti" Rokka; the politically indifferent Hietanen, and the communist Lahtinen.

Amidst a series of battles, the company assaults a Soviet bunker line on a ridge and stops an armoured attack, the ambushed and abandoned Lehto commits suicide during a regimental flanking maneuver, and the soldiers advance into East Karelia.

The period includes the soldiers drinking kilju (a home-made sugar wine) during Commander-in-Chief Field Marshal Carl Gustaf Emil Mannerheim's birthday celebrations and getting drunk, a new recruit being killed by a sniper for failing to listen to advice from experienced veterans and raising his head above the trench, and Rokka capturing an enemy captain during a nightly Soviet probe into the Finnish trenches.

The final act of the novel describes the defence against the Soviet Vyborg–Petrozavodsk offensive of summer 1944, the withdrawal and counter-attacks of the Finnish Army, and the numerous losses that the company suffers.

The company abandons their machine guns in a lake while withdrawing from a hopeless defence, and Lieutenant Colonel Karjula executes the retreating Private Viirilä in a burst of rage while trying to force his men into positions.

Väinö Linna wrote in his manuscript cover letter to the publisher WSOY that he wanted "to give the soldiers, who bore the weight of the calamity, all the appreciation and strip war of its glory".

The Herald described Linna's aim as "not to home in on individual plights and agendas but to show the whole great shapeless mass of a platoon, one that is continuously besieged and pared down".

[5][6][14] Aku Louhimies, director of the 2017 film adaptation, analysed Linna's intentions as follows: "I think [his] original idea was to show the events so that they would also act as a warning.

[2] The cover art of a soldier's white silhouette against a red background, designed by Martti Mykkänen, became similarly famous and is often used as a symbol for war in Finland.

For example, the disobedient but competent and pragmatic Rokka, or the humane jokester, Hietanen, are described as typical role models, while the calm, fair and composed Koskela is the paragon of every Finnish leader.