In 1943, with the purpose of intimidating neutral Turkey into joining their cause, the Axis powers set their sights on a group of 2000 British soldiers stranded on the island of Kheros in the Aegean Sea. The Royal Navy cannot execute a rescue due to powerful radar-guided guns located on the nearby island of Navarone. With time running out as a German offensive is imminent, the situation becomes increasingly desperate.
Efforts to neutralize the guns through bombing have failed, prompting Commodore Jensen of Allied intelligence to assemble a team of saboteurs tasked with the crucial mission of sailing to Navarone to take out these monstrous weapons. At the helm is Major Roy Franklin, portrayed by Anthony Quayle, with a skilled crew that includes Captain Keith Mallory, played by Gregory Peck, Andrea Stavrou, a former Colonel in the defeated Greek army, depicted by Anthony Quinn, and Corporal Miller, a former university chemistry teacher and explosives expert brought to life by David Niven. They are joined by Spyros Pappadimos, a Greek-American spy character portrayed by James Darren, and the no-nonsense engineer and fighter "Butcher" Brown, played by Stanley Baker.
Disguised as Greek fishermen aboard a rickety vessel, the team sets sail across the perilous waters of the Aegean. Their journey quickly turns hostile when they encounter a German patrol boat; in a tense confrontation, they manage to kill the German soldiers and sink the vessel. During this voyage, Mallory confides in Franklin about the promise made by Andrea to seek revenge for the perceived wrongdoings that led to the tragic loss of his family.
A fierce storm eventually wrecks their ship, resulting in lost equipment but allowing the team to safely land on the island. With Mallory, chosen for his exceptional climbing abilities, leading them, they embark on the arduous ascent of a seemingly unclimbable cliff. However, tragedy strikes when Franklin is grievously injured, later battling infection in the form of gangrene. As the dire circumstances unfold, Miller suggests leaving Franklin to the enemy's care, but Mallory, now in command, refuses, fearing he would divulge their mission details.
In a moment of despair, Franklin attempts to take his own life, yet Mallory deceives him by claiming that their mission has been cancelled and a grand naval assault is looming over Navarone. Throughout their perilous journey, they find local resistance allies, including Maria, portrayed by Irene Papas, Spyros's sister, and Anna, played by Gia Scala, who has pretended to be mute following her traumatic experiences under German torture.
As the team faces relentless pursuit from German forces, they are eventually captured by Lieutenant Muesel, portrayed by Walter Gotell, while seeking medical help for Franklin. Instead of giving in to interrogation, they plot an escape, with Andrea feigning betrayal to outsmart their captors. Regrettably, they must leave Franklin behind for medical assistance.
Eventually, Miller discovers that much of their explosives have been sabotaged and identifies Anna as the traitor. However, in a shocking twist, it becomes clear she is not mute and had voluntarily cooperated with the Germans. Maria ultimately takes matters into her own hands and executes her, marking a pivotal turn in the group's dynamics.
The team splits to tackle their final objectives: Mallory and Miller head for the guns, while Andrea and Pappadimos create a diversion. The duo infiltrates the fortified gun emplacements and sets off an alarm as they place explosives. A fierce chase ensues, leading to a daring escape into the sea.
As the Allied destroyers arrive on schedule, the mission heads to a climactic conclusion; despite the Germans managing to remove some explosives, the hidden charges are triggered, resulting in a tremendous explosion that signifies their hard-fought victory, echoing even to Franklin's hospital room. With the guns incapacitated and the team members reflecting on the upheaval their mission has caused, Andrea chooses to go back to Navarone with Maria, while Mallory and Miller watch from a distance, contemplating the true cost of their heroics.
