Bumbling former World War II serviceman Peter Stirling is sent to the U. S. Army's military academy at West Point as a reward for stopping a plot to blow up his government workplace. After enrolling, he is privately tutored by his old army friend Francis, which gets him into trouble when he reveals that this tutor is one of West Point's very own mule mascots.
Upon the United States entry into World War I, the first American units to arrive at the front in France are veteran Marine companies, one of which is commanded by captain Flagg, along with his Lieutenants, Moore and Aldrich. Flagg has developed a romantic relationship with the daughter of the local innkeeper, Charmaine, and resumes their relationship after returning from the front. However, he lies to her and tells her he is married when she wants to come with him on his leave to Paris. Replacements arrive and their lack of discipline and knowledge infuriate the Captain. But he is expecting the arrival of a new top sergeant, who he hopes will be able to train them properly. However, when the sergeant arrives, it is Quirt, Captain Flagg's long time rival, a rivalry which quickly re-ignites.
Gus Brubaker (Dick Shawn) is a self-described schnook. His wife talks him into applying for G.I. insurance for which he is eligible from his World War II service with the Air Force. Gus is reluctant because he was shot down and became a prisoner of war, but the military had listed him as killed. A red-tape foulup results in Gus being back in uniform, assigned to a ramshackle radar station on a backwater island near Japan. The airmen assigned there are apathetic, slovenly, and unmotivated. Its equipment and supplies are a collection of junk, abandoned or surplus.
In a hospital unit in the U.S. Army in Europe after World War II, Private Hogan (Jack Lemmon) does not believe that a blue-stocking can be good-looking, but the first sight of dietetic nurse Lieutenant Betty Bixby (Kathryn Grant) sets him straight. When he picks her cigarette lighter up and puts his weapon aside, he is surprísed by security officer Paul Locke (Ernie Kovacs) who admonishes him for putting down his weapon while on guard duty and confines him to quarters preliminary to a court martial. The Colonel in charge of the unit (Arthur O'Connell), however, would prefer to keep everything "in the family" and avoid a court martial.
Three decorated Navy pilots finagle a four-day leave in San Francisco. They land a posh suite at a hotel where Commander Andy Crewson (Cary Grant), a master of procurement, arranges to populate it with wine, women and song.
At Norfolk Naval Base in the opening months of World War II, Lieutenant John W. Harkness (Cooper), a newly commissioned officer, bids goodbye to wife Ellie (Jane Greer) and reports aboard the PC-1168 unaware that his civilian background in engineering and his Rutgers education has elected him, by means of a hole punched in an IBM card, to head a secret project and command the ship. The Navy has installed a steam engine and an experimental evaporator-condenser in the ship to test its feasibility in patrol craft and has assigned Harkness to conduct the sea trials.
A divorced socialite and daughter (Russell) of a United States senator (Douglas) asks her father to get her an officer's commission in the Women's Army Corps, so that she can be near her officer boyfriend.
Eddie Devane (William McNamara) is a young Seaman Apprentice who has carried out a number of inventory-related scams along with his partner-in-crime Howard (Crispin Glover) and made a lot of money during his service. A day before his discharge, Eddie is assigned to escort a prisoner from the marine base at Camp Lejeune along with the authoritarian, no-nonsense Chief Petty Officer Rock Reilly (Tom Berenger). Eddie is of course not pleased with this development. When Howard sees a grumpy-looking Eddie being escorted from his superior's office by a couple of other seaman, he thinks Eddie has been found out and arrested for his scams. In order to destroy evidence, he goes to Eddie's desk and finds the money, the existence of which Eddie had concealed from him.
Before the 1939 invasion of Poland by Nazi Germany, the stars of a theater company in Warsaw are the "ham" Josef Tura (Jack Benny) and his beautiful wife, Maria (Carole Lombard). As part of the company's rehearsal of a play satirizing the Nazis, one of the actors, Bronski (Tom Dugan), takes to the street to prove that he looks like Hitler in his costume and makeup. People gawk at the appearance of the Nazi dictator in Warsaw, until a young girl asks for the autograph of "Mr. Bronski."
When a visiting American senator decides to make Paris off-limits to enlisted military personnel, the daughter of the U.S. Ambassador to France decides to show him that American servicemen can be gentlemen by dating one of them without revealing her lofty social status. Sergeant Sullivan takes Joan to colorful nightclub cabarets, and on a comical trip up the Eiffel Tower, all the time believing her to be a Dior fashion model. Thinking she has an emergency back in America, Sullivan offers to buy her an airline ticket, for which she is grateful, until she hears that counterfeit plane tickets are a common scam used by American servicemen to impress girls. Sullivan's friend, the homespun Corporal O'Connor, all the while is a guest of the Ambassador's family and other top brass, and tries to alert Sullivan as to Joan's true identity, but is unable to contact Sulllivan (and is sworn to secrecy). When Sullivan drops into the Dior fashion show one day to look for Joan, the staff have never heard of her, however he sees her observing the show with her father's friend the Senator, whom he mistakenly assumes must be her sugar daddy. On their last dinner date, Joan walks out on Sullivan, when he accidentally spills wine on her and offers to take her to his hotel room, thinking he is dishonorable. Finally, one evening Sullivan and the Ambassador's family, by coincidence, separately attend the same ballet performance of Swan Lake, where during the intermission Sullivan learns her true identity and their misunderstanding is resolved.
Two guys from New York—Luigi, a hip wanna-be beatnik and Jerry, who's from Long Island—end up in Army basic training in New Jersey, as does Mike, who's a rancher from Oregon.
Lt Donald (Pat Boone) of the US Navy is assigned to look after a troublesome sailor, Garfield (Buddy Hackett), who is from an oil-rich Indian tribe. He also romances journalist Sally Hobson (Barbara Eden).
Andy Schaeffer is a spoiled mama's boy who usually gets his way. He breezes through college while girlfriend Susan Daniels studies hard while also working at a job to pay for her education.