Captain John Yossarian, the novel's protagonist, is a bombardier in the 256th Squadron of the Army Air Forces during World War II, stationed on Pianosa, a fictionalized island in the Mediterranean between mainland Italy and Corsica. The squadron's assignment is to bomb enemy positions in Italy and eastern France. It is the summer of 1944, but events of the novel are not told in chronological order; sometimes time changes without warning. Most of the action takes place from spring through December of 1944, but there are flashbacks to 1942, when Yossarian was in basic training at Lowery Field in Colorado, and to 1943 when he was in cadet training at Santa Ana, California. The satirical novel is episodic and relies on character as much as it does on plot or theme.
We meet several key characters in the opening chapters. When the novel opens, Yossarian is in the base hospital, on Pianosa, faking a liver ailment. He has learned that the hospital can be a peaceful refuge and that liver ailments are difficult to diagnose. When able, hospitalized officers censor enlisted men's letters home; an early indication of Yossarian's character is the creative way he approaches this task, even altering one letter to resemble a love note and signing Chaplain Tappman's name. Others in Yossarian's ward include Dunbar, who thinks he can live longer by pursuing boring tasks and making time pass slowly, and the "soldier in white," an enigmatic figure smuggled into the ward in the middle of the night and encased from head to toe in gauze and plaster. Yossarian is so annoyed by a bigoted Texan that he soon returns to his regular quarters where his tent mate, Orr, uses outstanding mechanical talents to improve living conditions. Other squadron members include McWatt, a pilot who enjoys buzzing (flying low over) Yossarian's tent and the beach bathers; Nately, who is in love with a prostitute in Rome; Doc Daneeka, a friendly but ineffectual flight surgeon who explains the meaning of "Catch-22" (Chapter 5); and Colonel Cathcart, who yearns to be a general so he tries to look like a strong leader by raising the number of missions required of the men to complete a tour of duty.
The early chapters satirize the military and political establishment, specifically through distortions of logic and reality. Earlier in the summer, when an apparently hazardous raid on Bologna seems imminent, for example, Yossarian postpones the mission by simply moving a line on a map, indicating that the Americans and other Allies have already captured the city. Although war and death are always in the background, the tone of the first third of the novel is ironic and absurdly light-hearted.
The tone changes with flashbacks to raids on Ferrara, Bologna, and Avignon. At Ferrara, in the spring of 1944, Yossarian is an inexperienced bombardier eager to succeed in the mission so it will not be repeated. Having failed to release his bombs on the first run, Yossarian directs his flight to take a second pass so that he can hit the target, a bridge that the squadron has tried in vain to destroy for a week. The tactic works, but a young airman named Kraft is killed. Although Yossarian is promoted to Captain and awarded a medal, the death haunts him. When the squadron eventually must bomb Bologna a second time, in late June 1944, the flak (ground fire at the aircraft) is exceptionally intense. Heller's description of the raid (Chapter 15) is one of the most vivid passages in the novel. By now, Yossarian has had all he wants of war. His tent mate, Orr, is shot down and presumed lost at sea. The mission over Avignon, in July, is personally even more traumatic for Yossarian as his plane is badly hit and a gunner named Snowden severely wounded. Heller repeatedly refers to the event throughout the novel, playing it like a recurring theme in a symphony, the reader allowed to learn a little more of the horror as the story progresses. (The novel's first scene occurs shortly after the raid on Avignon.)
Leaves of absence in Rome allow respite for the men and a change of tone for the reader. Here the men initially find romance, parties, frolic, and joy. Yossarian's brief affair with a young woman named Luciana is one of the more poignant interludes, although we soon are reminded that Yossarian's relations with women are troubled at best. Nately argues patriotism with a 107-year-old cynic at a bordello, one of the novel's several debates concerning values. But Rome, too, is altered by the war, as Yossarian learns in the closing chapters.
As the story progresses, amusing antics turn grim. The squadron's mess (dining hall) officer is an entrepreneurial whiz named Milo Minderbinder, whose capitalistic expertise is satirically entertaining until it turns deadly. Using squadron funds to purchase black-market products, Milo builds an enormous syndicate dealing in everything from fresh eggs to prostitutes. He eventually contracts with both sides in the conduct of the war and goes so far as to arrange an air raid on his own base for profit. McWatt's buzzing of the beach is a practical joke until one day when he slices Kid Sampson in two in a macabre accident. Nately's devotion to his prostitute seems like an innocent young man's harmless crush until he insists on flying extra missions so that he can stay near her — and is killed the next time out.
Nately's death serves as a transition to the end of the novel. When Yossarian reports the tragedy to the young man's prostitute-fiancée in Rome, she directs all her frustration and anger toward Yossarian, attacking him repeatedly, even after he returns to Pianosa. Yossarian has refused to fly further missions. Colonels Cathcart and Korn call him in for a conference in which they offer Yossarian a "deal": He can return home if only he will speak well of the commanding officers and turn his back on the men of the squadron. Yossarian agrees. Leaving the office, he is again assaulted by Nately's fiancée and seriously wounded. In the hospital, Yossarian reflects on all his friends who have died or disappeared during the year. He decides to renege on the agreement even though he is thus eligible for court-martial. Yossarian's plan is to rescue Nately's kid sister from the streets of Rome and flee to Sweden where his buddy Orr has turned up after cleverly using his plane crash as a means of escape. Yossarian hopes to find himself by losing himself, to seek a separate peace, to run toward life by escaping the madness of war.