As the film opens, a 12-year-old boy is running through an open field laughing and strolling in a carefree manner. As he runs around, he encounters his mother and informs her that there is a cuckoo in a tree. The boy, Ivan Bondarev (Nikolay Burlyaev) suddenly wakes up, revealing that the previous scene was a dream. He currently spends his nights in a windmill and leaves the building to explore. The windmill is in the middle of an open field with abandoned farm equipment with smoke swirling around. Next, the boy is wading through marshes and tall grasses as the sun is setting. While he wades through, flares are sent up and he encounters barbed wire. The film cuts to the title screen.
A Russian officer wakes up Comrade Senior Lieutenant Galtsev (Yevgeni Zharikov) in the middle of the night because they found a boy crossing the Dnepr River. Ivan, wet and shivering, asks the informer to leave and declares himself Bondarev, asking for Number 51 at headquarters. Galtsev makes the first call and is ridiculed by the man on the phone. When he informs Bondarev that he will need to be locked up, Ivan insists that he calls again and even provides Galtsev with the code address of VCH 49550. When Galtsev gets through Number 51, he is instructed to provide the Ivan with paper and a pen and send him anything that the boy writes down. Galtsev realizes the importance of the boy and follows the orders in addition to providing the boy with food and a bath. Ivan counts pine needs and seeds, displaying a code and seals the envelope for it to be delivered. After, Ivan takes a bath and eats his food before falling asleep.
As the sound of water drips in the barrack, Ivan dreams of his mother and himself dropping a feather into a well. During this dream, his mother and he speak of a star that remains in the well. Later, Ivan is in the well reaching for the allusive star as his mother is shot, presumably by German soldiers. Ivan awakes to see that Galtsev is still in the room. Ivan admits to becoming jittery and inquires whether he talks in his sleep. As this conversation ends, Captain Kholin (Valentin Zubkov) enters the room and Ivan and Kholin share a warm, familial embrace. Ivan is informed that Corporal Katasonov is still waiting for him at Hollow Tree but is surrounded by Germans. Ivan informs Kholin that because he is so small, he was able to get away and swim across the river. Kholin is not pleased with this fact, especially because Ivan has lost so much weight; this scene displays how much Kholin cares for Ivan and treats him like a son. At this point, Kholin provides Ivan with clothing.
In the next scene, Ivan is informed that he will be sent to military school and disapproves of the decision stating that he must continue to fight the Germans. Because Kholin and Katasonov have taken such a liking to Ivan, the wish to see him succeed and leave the front lines. Due to Ivans disapproval, he runs away to the countryside where he finds an old man (Dmitri Milyutenko). After a few minutes of conversing, a Russian car comes speeding up to retrieve Ivan. Ivan reluctantly returns to the car but threatens that he will continue to run away if they dont send him back across the river. As the car leaves, Ivan leaves a loaf of bread and a can of food for the old man.
Later, the film cuts to the nearby forest where Kholin is aggressively pursuing a romantic relationship with Masha (Valentina Malyavina), the doctors assistant. Masha does her best to refuse the advances of the Captain but he forces a kiss and a hug on her. Afterward, as she walks away, an old friend that she took her college entrance exams with stops her. As they are speaking, Kholin, Galtsev, and Katasonov walk by and receive the attention of Masha. Masha leaves her old friend and wanders into the woods.
After Ivan spends three days at headquarters, he returns to the barracks where Galtsev tries to give him various magazines to read to occupy his time. Unfortunately, Ivan had already read all of that material during the last three days. Ivan takes interest in an art book. While showing Ivan the art book, Galtsev overhears Kholin and Katasonov making plans to take Ivan on another mission across the Dnepr River. Galtsev offers to join, but they do not need him. Ivan looks through Galtsevs belongings and finds a knife that once belonged to Galtsevs friend. When Ivan asks for the knife, Galtsev reluctantly gives him the weapon.
Later, outside the trenches, Galtsev inquires about what will happen to Ivan once the war is over. Kholin informs him that Katasonov or the Colonel will adopt him. The scene cuts to Ivan who sets up a practice hunt of German soldiers in order to avenge his familys murder. During this, a real German air raid begins. Galtsev returns to the barracks where he checks on Ivan, cleans up the room, and is informed by Ivan that he wants to continue to fight.
Kholin returns to Ivan later that day and informs Ivan that Katasonov was sent back to headquarters before he could say goodbye to Ivan. Instead, Ivan will be accompanied by Kholin and Galtsev across the river. The three eat dinner and Ivan falls asleep. During this time, he dreams of the time when he rode atop an apple truck with his sister in the rain as the truck drove onto the beach. As the apples fall out of the truck on the beach, horses eat the fallen apples, and Ivan and his sister enjoy the rain on their skin. During this dream, Kholin wakes Ivan in order to get ready for the night. While getting ready, Ivan expresses his anger that Katasonov did not visit him prior to his departure. The three get dressed and prepare to leave to cross the river. Kholin advises Galtsev to make a diversion while he and Ivan get to the boat. While making the diversion, Galtsev discovers that Katasonov was shot and they are hiding it from Ivan.
Kholin, Ivan, and Galtsev row across the Dnepr River, wade through the marshes, and eventually say their goodbyes as Ivan must begin his crossing. Once Ivan left, Kholin and Galtsev row back across the river, meet some artillery fire, but get back safely to the barracks. Once there, they play the gramophone only to be interrupted by Masha as she says goodbye to Galtsev.
Later, the scene is in Berlin after Russia has won the war. As troops sift through the paperwork, a scarred and war-torn Galtsev finds Ivans file and discovers that he was captured and executed. The film ends with Ivans last dream when his mother leaves him to play with other children on the beach and he chases his sister through the water.