As the film opens in 1962, General Douglas MacArthur (Gregory Peck) is at West Point, his alma mater. MacArthur, ramrod straight, reviews the ranks of marching cadets before delivering his poignant, bittersweet, farewell address to the Corps.
The action abruptly shifts back to the tunnels of Corregidor in early 1942, where MacArthur's besieged American and Filipino troops are desperately holding out against the victorious forces of Japan. The end is near.
In Washington, General George Marshall and Admiral Ernest J. King are simultaneously delivering the grim news to President Roosevelt (Dan O'Herlihy). They discuss global priorities and admit that they can't hold the Philippines. Roosevelt reluctantly decides to order MacArthur off Corregidor, knowing that he will be furious. MacArthur is ordered to board a submarine and slip away to Australia, where he can organize ongoing resistance. But MacArthur refuses to sneak away from his beloved command traveling furtively beneath the waves. He insists on boarding a PT boat, and his family insists on accompanying him on the dangerous voyage. The Japanese navy has thrown a blockade around the islands, and the heavily armed PT boats would be no match for destroyers. The pair of PT boats manage to evade the Japanese pickets and floating mines, proving that the blockade can be circumvented.
Eventually arriving in Melbourne, MacArthur discovers that there is no Philippine relief expedition being organized. He addresses the adoring Australian crowds, explaining that his first priority will be the recapture of the Philippines. But he soon learns that Bataan and Corregidor have fallen, leaving 70,000 of his starving men imprisoned under unimaginable conditions.
MacArthur decides to mount the defense of Australia among the jungles of New Guinea, where the battle is fought under brutal circumstances. As a result, MacArthur develops his famous island hopping strategy, bypassing Japanese strongholds through the use of Allied naval and air superiority, cutting critical supply lines and starving the isolated Japanese strong points into submission. MacArthur also develops a well-earned reputation for dramatics, self-promotion and vanity.
The Allies advance steadily up the Southwest Pacific island chain over the next two years, but not without tremendous bloodshed. Ordered to Honolulu in July 1944 to meet with President Roosevelt and Admirals Leahy and Nimitz, MacArthur upstages his naval rivals with a grandiose, belated arrival. His jealous, distrustful relationship with Roosevelt is apparent. The US Navy wants to bypass the Philippines and take Taiwan first. MacArthur makes an impassioned argument that to do so would be an indelible stain on American honor. He reminds them that numerous promises have been made to the loyal people of the Philippines, including by Roosevelt himself. In the end, MacArthur's view prevails.
US forces come ashore at Leyte Gulf in October 1944. In an iconic scene, MacArthur insists on going ashore right behind the second wave of troops. He steps off the landing craft into knee-deep water, wading ashore as the cameras roll. On the beach, MacArthur addresses the Filipinos by radio, exhorting them to drive the Japanese occupiers out. As the battle rages, the General receives his fifth star. He is soon taken to a prison camp where many of the brutalized, emaciated survivors of Bataan and Corregidor have been held for more than two years. Many emotional reunions follow.
Back home, President Roosevelt dies in the spring of 1945. Harry Truman succeeds him and authorizes the use of two atomic weapons to bring Japan to her knees. A furious MacArthur rails against this new, impersonal brand of warfare, realizing that an era has passed.
Aboard the USS Missouri in Tokyo Bay, MacArthur presides over the solemn surrender ceremonies in September 1945.
As Allied military governor of Japan, MacArthur institutes liberal social reforms and oversees the complete rebuilding and modernization of Japan. When the Russian military representative in Japan curtly informs MacArthur that Russia will soon commence an illegal occupation of Hokkaido Island, MacArthur just as curtly informs the Russian that the entire Soviet delegation will be thrown in jail. The occupation never occurs.
MacArthur is given one last chance at military glory in June 1950 when Communist North Korean forces invade South Korea, moving swiftly and nearly pushing American and South Korean forces off the peninsula. MacArthur is appointed Supreme Commander of UN forces in Korea. He buys time until he can devise a brilliant but risk-laden amphibious landing behind North Korean lines at Inchon. The successful operation proves to be a stroke of genius. North Korean forces are virtually cut off from supplies and reinforcements and have to retreat northward in disarray. MacArthur then carries the fight into North Korea itself. Despite MacArthur's reassurances to the contrary, the Communist Chinese suddenly enter the war in huge numbers in order to bolster their sagging North Korean ally. UN forces are again pushed southward, incurring heavy casualties. MacArthur grows increasingly frustrated, chafing at political interference from Washington.
Tensions between Truman and MacArthur boil over when MacArthur's insubordination in the face of direct orders threatens to undermine Truman's authority. After a stellar military career spanning more than a half-century, MacArthur is relieved of command.
He returns to a hero's welcome in April 1951 while Truman's popularity plummets. MacArthur delivers a moving retirement address to a joint session of Congress. The movie then moves forward to 1962, and an increasingly frail Douglas MacArthur is bidding farewell to West Point. One last time he utters the famous words that were the hallmark of his career: Duty. Honor. Country.