A disclaimer appears at the beginning of the film, acknowledging that it is fiction: "This is not a documentary of the war in Korea, but a dramatized story of the effect of war on a group of people. All persons other than those whose real names are used in this film are fictitious and any similarity between them and any persons living or dead is purely coincidental. Where dramatic license has been deemed necessary, the authors have taken advantage of this license to dramatize the subject."
The film depicts the Battle of Inchon during the Korean War. The protagonist of the film is General Douglas MacArthur (Laurence Olivier), who led the United States surprise amphibious landing at Incheon on September 15, 1950. A subplot involves a young American couple who have problems in the mist of this war.
Inchon begins with a depiction of North Korean soldiers moving past the 38th parallel north into South Korea in June 1950. Residents of South Korea flee into the country's capital, Seoul. A United States Major's wife, named Barbara Hallsworth (Jacqueline Bisset), arrives in a village located at the 38th Parallel where she is attempting to buy antique furniture and items for her business as an interior decorator. She hears a bulletin over the radio saying: "The Communists are coming", and decides to leave the village. A limousine driven by a chauffeur takes her to Seoul where she encounters a group of five Korean children. Barbara decides to take the five kids with her away to a safe haven across the country after Seoul falls to the Communists. During the long journey, Barbara's chauffeur dies during the North Korean invasion when they barly escape from an ambush. Barbara takes over driving and drives them to a safe location called the "Inn of the Sixth Happiness". Along the way, she kills a North Korean soldier by shooting him between the eyes.
Meanwhile, Barbara's husband, U.S. Major Frank Hallsworth (Ben Gazzara) is attempting to break off a secret affair with a young Korean woman (Karen Kahn). The woman's father Saito-san (Toshiro Mifune) is aware of his daughter's affair with Hallsworth and does not disapprove of it. Hallsworth receives word of the invasion by the North Koreans, and he travels north in an attempt to locate his wife with the assistance of army sergeant August Henderson (Richard Roundtree). Henderson encounters Barbara after she dropped off the five children in her care and fixes her vehicle's battery, and then reunites her with her husband.
Meanwhile, David Feld Park (David Janssen), an American journalist in Tokyo, is waiting with other reporters for a press conference to begin which will be held by General MacArthur. Longfellow (Rex Reed), a second reporter awaiting the press conference, is generally a music critic by also reports on serious events developing in Tokyo. General MacArthur is at his residence in Tokyo with his wife, and does not appear at the press conference. He agrees with his wife that he is the only individual who can rescue South Korea from the communist invasion by the North Koreans.
Back in South Korea, Major Hallsworth and his former lover hide out in Inchon where they succeed in turning on a lighthouse to signal an invasion fleet of 261 U.S. ships, and the Korean woman's father, (in league with the North Koreans) activates mines in the channel. She dies during the ensuing battle as the U.S. troops land, and a huge climatic gun battle begins. After a long and violent battle, American troops drive out the North Korean forces and restore President Syngman Rhee (Kwang Nam Yang) to power. General MacArthur gives Rhee a hug, as people wave South Korean and American flags.
In the final scene, we see MacArthur saying the Lord's Prayer in front of a bust of Julius Caesar. After this scene, an actual newsreel of the real Douglas MacArthur is displayed.