During the American Civil War, the Massachusetts Infantry Regiment meets Confederate forces in the devastating Battle of Antietam. Captain Robert Gould Shaw, portrayed by Matthew Broderick, is injured during the combat and initially presumed dead. He is later discovered alive by a gravedigger, John Rawlins played by Morgan Freeman, who takes him to a field hospital. While recuperating in Boston, Shaw reunites with his family and meets the former slave and abolitionist, Frederick Douglass (Raymond St. Jacques). Shaw is then offered a promotion to Colonel and the daunting responsibility of leading the first all-black regiment, the 54th Massachusetts Volunteer Infantry, composed entirely of black soldiers. Accepting this opportunity, he enlists his childhood friend, Major Cabot Forbes, portrayed by Cary Elwes, as his second-in-command.
Shaw's first recruit is another friend of his, the scholarly freeman Thomas Searles (Andre Braugher). Other soldiers soon join the cause, including Rawlins, a timid freeman named Jupiter Sharts (Jihmi Kennedy), and Silas Trip, portrayed by Denzel Washington, an escaped slave who is initially suspicious of Shaw. Tensions arise between Trip and Thomas, with Rawlins stepping in to maintain peace among the men.
As the Confederacy issues a stark warning that any captured black soldiers and their white officers will face execution, the opportunity for honorable discharges is presented to the 54th. Surprisingly, not a single man steps back. The soldiers endure a brutal training regimen, overseen by Sergeant Mulcahy (John Finn). Mulcahy's strict treatment particularly affects Thomas, who has enjoyed a relatively easier life compared to his comrades and struggles with the physical demands. Shaw voices his concern for Thomas, but Mulcahy argues that all soldiers must endure the same training. Ultimately, Shaw recognizes that his men's eagerness to fight necessitates equal treatment, even if it creates friction between him and Forbes, who doubts whether the 54th will see real combat.
When Trip goes AWOL and is caught, Shaw is faced with a moral dilemma as he orders his flogging in front of the regiment. Trip's scars from his past life as a slave are revealed, challenging Shaw's principles as an abolitionist. In a revealing conversation with Rawlins, Shaw discovers that Trip's absence was driven by his search for decent shoes to replace his worn-out pair. This revelation leads Shaw to confront the quartermaster, Kendric (Richard Riehle), learning that the troops are being denied essential supplies purely because of their race. Shaw's advocacy for his soldiers continues as he fights for equal pay, following a controversial decision by the Federal government to compensate black soldiers less than their white counterparts. This leads to a protest organized by Trip, with Shaw himself joining the cause for fairness, culminating in the soldiers donning their Union uniforms, which instills a newfound pride within the regiment.
Once training is completed, the 54th is assigned to General Charles Garrison Harker (Bob Gunton) and is paired with the 2nd South Carolina Volunteer Infantry Regiment, largely made up of freed slaves and commanded by Colonel James Montgomery (Cliff De Young). During their transport, Rawlins is promoted to Sergeant Major. Passing through Darien, Georgia, Montgomery's orders lead to chaos and destruction as he allows his undisciplined men to loot the town. Amid escalating tensions, Shaw is coerced into illegal actions, forced to comply under threats regarding his command and the treatment of his men.
Despite various responsibilities that have kept them from active duty, Shaw persistently pushes for opportunities for his men to fight. His demands eventually lead them into battle on James Island, South Carolina. Initially, the 54th experiences a surge of success, but they soon face fierce opposition, resulting in significant casualties. During this confrontation, Thomas is wounded but heroically saves Trip, finally earning his respect. Although offered medical leave, Thomas chooses to stay with his comrades.
After some time, General George Strong (Jay O. Sanders) informs Shaw of a critical campaign aimed at securing Charleston Harbor, which necessitates an assault on the heavily fortified Battery Wagner. The assault's approach is treacherous, with a narrow strip of beach offering little protection, marking a certain path of severe losses. Shaw volunteers for the mission, leading the charge for the 54th.
The night before the battle, the regiment unites for a religious service, filled with emotional speeches and hymn singing, as they seek divine guidance and strength. When the charge is launched, the soldiers face overwhelming fire, leading to catastrophic losses on the beach. Amidst the chaos and heart-wrenching scenes, Shaw encourages his men to advance, ultimately sacrificing his life after sustaining multiple gunshot wounds. In a moment of valor, Trip raises the flag, rallying his fellow soldiers even as he suffers grievous injuries himself.
Forbes steps into leadership as the regiment pushes forward, yet they find themselves outnumbered within the fort after breaching the initial defenses. The aftermath unveils a chilling sight: the beach strewn with the bodies of fallen soldiers, and the Confederate flag raised above the fort as a grim reminder of their sacrifice. As the soldiers are buried in a mass grave, Shaw and Trip's bodies lie side by side, symbolizing their shared fate.
In the end, the closing narration reveals that Battery Wagner was never taken by Union forces. However, the heroic actions of the 54th, which endured staggering losses, were not in vain; their bravery became a catalyst for increased recruitment of black soldiers within the Union army. Their contributions would later be acknowledged by President Abraham Lincoln as pivotal to securing the Union's ultimate victory over the Confederacy.
