As a child, Desmond Doss nearly kills his younger brother Hal. This event and his Seventh-day Adventist upbringing reinforce Desmond’s belief in the commandment “Thou shalt not kill”. Years later, Doss takes an injured man to the hospital and meets a nurse, Dorothy Schutte. The two strike a romance and Doss tells Dorothy of his interest in medical work.
After the Japanese attack on Pearl Harbor, Doss enlists in the Army to serve as a combat medic. His father Tom, a World War I veteran, is deeply upset by the decision. Before leaving for Fort Jackson, he asks for Dorothy’s hand in marriage and she accepts.
Doss is placed under the command of Sergeant Howell. He excels physically but becomes an outcast among his fellow soldiers for refusing to handle a rifle and train on Saturdays. Howell and Captain Glover attempt to discharge Doss for psychiatric reasons but are overruled, as Doss’ religious beliefs do not constitute mental illness. They subsequently torment Doss by putting him through grueling labor, intending to get Doss to leave of his own accord. Despite being beaten one night by his fellow soldiers, he refuses to identify his attackers and continues training.
Doss’ unit completes basic training and is released on leave, during which Doss intends to marry Dorothy, but his refusal to carry a firearm leads to an arrest for insubordination. Captain Glover and Dorothy visit Doss in jail and try to convince him to plead guilty so that he can be released without charge but Doss refuses to compromise his beliefs. At his trial, Doss pleads not guilty, but before he is sentenced, his father barges into the tribunal with a letter from a former commanding officer stating that his son’s pacifism is protected by an Act of Congress. The charges against Doss are dropped, and he and Dorothy are married.
Doss’ unit is assigned to the 77th Infantry Division and deployed to the Pacific theater. During the Battle of Okinawa, Doss’ unit is informed that they are to relieve the 96th Infantry Division, which was tasked with ascending and securing the Maeda Escarpment (“Hacksaw Ridge”). In the initial fight, both sides sustain heavy losses. Doss saves his squadmate Smitty as well as several wounded soldiers. The Americans camp for the night, which Doss spends in a foxhole with Smitty. Doss reveals that his aversion to holding a firearm stems from nearly shooting his drunken father, who threatened his mother with a gun. Smitty apologizes for doubting his courage, and the two reconcile.