The film unfolds in two distinct segments, beginning with the journey of four university students—Hoang, Thanh, Thang, and Long—as they enlist in the military, and progressing to the fierce Battle of the Quang Tri Citadel. Initially, we witness the moment the four young men receive their enlistment orders. In a heartfelt gathering at Thong Nhat Park, they capture their camaraderie with a commemorative photo, laughing and posing near a statue of a girl engrossed in a book. The photographer, portrayed by People's Artist Nguyen Hai, turns down payment, instead promising to take a follow-up photo once they return victorious.
As they prepare to leave, the friends visit their lecture hall at the University of Hanoi to write farewell messages on the blackboard. Long faces a personal struggle; after witnessing his parents’ divorce in court, he decides to remove the curtain that separates their room and pushes their beds together, leaving them an emotional letter behind. Each friend enlists in succession, and during their journey to the recruitment base, Hoang plays music while the recruits humorously sing a parody of the song Footsteps on the Truong Son Range:
> "We are our parents' children
> If we miss home, we'll just slip away
> We need no rucksack, no dry rations, no mess tin
> We'll go home for a few minutes, then come right back..."
Their Company Commander, Phong, takes a firm stance, lecturing the young soldiers about military discipline as they travel. Once at the barracks, they endure rigorous training, but they also share precious moments of youthful joy—sneaking out, reading poetry, singing Chèo opera, bathing freely, and enjoying playful pranks. Their rucksacks still contain tokens from home, like metal cicada toys and colorful marbles. During a stint in a nearby village, Long falls for a local girl washing clothes by the well; infused with emotion, he conveys his feelings through music. On deployment day, the village girl gifts Long an embroidered handkerchief, wrapped in a clover-patterned case inscribed with "Souvenir 1971," and promises to wait for his return.
As they approach the front lines, tragedy strikes when Hoang is wounded by artillery fire, which prevents him from participating in the initial offensive along with his friends. Once the Battle of the Quang Tri Citadel commences, chaos reigns as soldiers wade across the Thach Han River, suffering terrible losses. Out of 107 men, only 49 survive the crossing, emerging into a horrific scene adorned with the wounded and body bags. Long, facing the grim realities of war, succumbs to panic and is tragically killed by shrapnel. His comrades bury him alongside a tattered curtain, a charred guitar, and the blood-stained handkerchief, only for the fresh grave to be destroyed soon after by artillery fire.
As the battle rages within the citadel, the young Liberation soldiers encounter elite units from the Army of the Republic of Vietnam, bolstered by heavy artillery and air support. The intensity of the battle escalates, especially amidst the ongoing Paris Peace Talks. During brief respites from the fighting, the young soldiers steal moments to share jokes and play; Thanh even dons a woman's costume to perform a role in Chèo opera while smoke swirls around them. After recovering from his injuries, Hoang manages to reach the citadel, reuniting with Thanh and Thang, though they are now part of separate units. Heartbreakingly, Thang is killed by enemy fire while trying to fix a severed communication line.
As the battle reaches its zenith, the Army of the Republic of Vietnam amplifies its assault, intent on hoisting its flag atop the ancient citadel. In a frenetic attempt to thwart the enemy advances, Thanh and his fellow soldiers discover their ammunition is depleted, leading to devastating losses. In an act of bravery and desperation, Thanh charges forward with his bayonet-mounted AK-47, successfully killing the soldier bearing the yellow-and-red-striped flag, but ultimately falls victim to enemy fire himself. Each loss of the four friends triggers the statue of the girl in Thong Nhat Park to weep tears of blood.
The film closes with the Spring General Offensive of 1975, culminating in a powerful scene depicting a tank from the Vietnam People's Army crashing through the gates of the Independence Palace. Hoang is seen reuniting with Company Commander Phong in the palace courtyard, where the two embrace in a poignant moment of sorrow. In a symbolic gesture, Phong returns the photograph taken before their enlistment, showcasing the unbreakable bond of friendship that they shared amidst the turmoil of war.
