The film depicts the Battle of the Alamo and the events leading up to it. Sam Houston leads the forces of Texas against Mexico and needs time to build an army. The opposing Mexican forces, led by General Santa Anna, are numerically stronger as well as better-armed and -trained. Nevertheless, the Texans have spirit and morale remains generally high. Lieutenant Colonel William Travis is tasked with defending the Alamo, a former mission in San Antonio. Jim Bowie comes with reinforcements and the defenders prepare. Meanwhile, Davy Crockett arrives with a group of Tennesseans.
Santa Anna’s armies arrive and surround the fort. The siege begins. An embassy from the Mexican Army approaches the Alamo, and as they list the terms of surrender, Travis fires a cannon, signalling his refusal to surrender. In a nighttime raid, the Texans sabotage a super-sized cannon used by the Mexicans. They maintain high hopes as they are told a strong force led by Colonel James Fannin is on its way to break the siege. Crockett, however, sensing an imminent attack, sends one of his younger men, Smitty, to ask Houston for help, knowing this will perhaps save Smitty’s life.
The Mexicans frontally attack the Alamo. The defenders hold out and inflict heavy losses on the Mexicans, although the Texans’ own losses are not insignificant, and Bowie sustains a leg wound. Morale drops when a messenger informs Travis that Fannin’s reinforcements have been ambushed and slaughtered by the Mexicans. Travis chooses to stay with his command and defend the Alamo, but he gives the other defenders the option of leaving. Crockett, Bowie and their men prepare to leave, but an inspired tribute by Travis convinces them to stay and fight to the end. The noncombatants, including most of the women and children, leave the Alamo.
On the thirteenth day of the siege, Santa Anna’s artillery bombards the Alamo, and the entire Mexican army sweeps forward, attacking on all sides. The defenders kill numerous Mexicans, but the attack is overwhelming and the fortress’ walls are breached. Travis tries to rally the men, but is shot and killed. Crockett leads the Texans in the final defense of the fort, but the Mexicans swarm through and overwhelm the defenders. Crockett is killed in the chaos when he is run through by a lance and then blown up as he ignites the powder magazine. Bowie, in bed with his wound, kills several Mexicans but is bayoneted and dies. As the last Texan is killed, the Mexican soldiers discover the hiding place of the wife and child of Texan defender Captain Dickinson.
The battle eventually ends with a total victory for the Mexicans. Santa Anna observes the carnage and provides safe passage for Mrs. Dickinson and her child. Smitty returns too late, watching from a distance. He takes off his hat in respect and then escorts Mrs. Dickinson away from the battlefield.
The subplot follows the conflict existing among the strong-willed personalities of Travis, Bowie, and Crockett. Travis stubbornly defends his decisions as commander of the garrison against the suggestions of the other two – particularly Bowie with whom the most bitter conflict develops – as well as trying to maintain discipline among a force made up primarily of independently minded frontiersmen and settlers. Crockett, well liked by both Bowie and Travis, eventually becomes a mediator between the other two as Bowie constantly threatens to withdraw his men rather than deal with Travis. Despite their personal conflicts, all three learn to subordinate their differences, and in the end, bind themselves together in an act of bravery to defend the fort against inevitable defeat.