Captain Keith Mallory was a New Zealand climbing expert. He was said to be capable of climbing almost anything, and his nickname was "The Human Fly." He served in the British military in the Long Range Desert Group during World War II. Because of mountaineering abilities, he was recruited by Captain James Jensen to accompany Lieutenant Andrew Stevens' team to Navarone to sabotage the great guns there. Mallory's expertise was used to get the team up an unguarded cliff thought to be impossible to climb. When Stevens slipped and was badly wounded in the climb, Mallory became the de-facto leader of the team. He and Corporal Dusty Miller eventually succeeded in destroying the Navarone guns, despite being captured by Lieutenant Turzig briefly and being betrayed by their Greek ally Panayis. After successfully completing their mission, he and the survivors of his team, Miller, his friend Andrea Stavros, Casey Brown and Greek resistance member Louki, were rescued by the Royal Navy ship the Sirdar.
Mallory was thereafter flown to Italy along with Andrea and Miller to meet with Jensen, where he was assigned to go to Yugoslavia to rescue several British officers held prisoner by the Germans, and also to destroy an important bridge to prevent the forces of General Zimmermann from wiping out the Partisan troops led by General Vukolovic. To aid him, Mallory was given three Royal Marine commandos, Sergeant Reynolds, Sergeant Groves and Sergeant Saunders. The six men flew into Yugoslavian airspace and feigned a crashlanding, where they surrendered themselves to the Cetnik troops led by Captain Droshny, pretending to be defectors. Droshny didn't believe they were defectors, and took them to his German superior, Captain Neufeld, who, after some convincing, believed they were.
Under the pretense of assisting the Cetniks defeat the Partisans and root out their secret leader, Mallory and his team went to the nearby Partisan camp along with blind musician Petar and his sister Maria. It turned out that Petar was the secret leader of the Partisans, although neither Neufeld nor Droshny knew it. While they were in the Partisan camp, Droshny, who still didn't trust Mallory, snuck in and murdered Saunders. Eventually, Maria and Petar were captured by Neufeld and the Cetniks, however Mallory and his team returned to the Cetnik camp and rescued them, and forced Neufeld and Droshny to take them to where the captured British officers were being held.
They freed the British prisoners, locking Neufeld and Droshny up in their place. The rescued British officers were flown out of the country on a plane, while Mallory and the others remained so they could destroy the bridge and prevent General Zimmermann from wiping out the Partisans. Despite interference from Neufeld and Droshny, who were freed, the team made it to a dam upriver from the bridge. They fought the German guards, whilst Miller planted his bombs and Andrea fought and killed Droshny. During the battle, both Reynolds and Groves were killed, but the dam was successfully blown, sending a tidal wave which wiped out the bridge, and Zimmermann's armored column with it. Following the completion of their mission, Mallory, Andrea and Miller returned to Italy, where, to their annoyance, Jensen had another assignment for them!