Joe and Kate Keller had two sons, Chris and Larry. Keller owned a manufacturing plant with Steve Deever, and their families were close. Steve's daughter Ann was Larry's beau, and George was their friend. When the war came, both Keller boys and George were drafted.
During the war, Keller's and Deever's manufacturing plant had a very profitable contract with the U.S. Army, supplying airplane parts. One morning, a shipment of defective parts came in. Under pressure from the army to keep up the output, Steve Deever called Keller, who had not yet come into work that morning, to ask what he should do. Keller told Steve to weld the cracks in the airplane parts and ship them out. Steve was nervous about doing this alone, but Keller said that he had the flu and could not go into work. Steve shipped out the defective but possibly safe parts on his own.
Later, it was discovered that the defective parts caused twenty-one planes to crash and their pilots to die. Steve and Keller were arrested and convicted, but Keller managed to win an appeal and get his conviction overturned. He claimed that Steve did not call him and that he was completely unaware of the shipment. Keller went home free, while Steve remained in jail, shunned by his family.
Meanwhile, overseas, Larry received word about the first conviction. Racked with shame and grief, he wrote a letter to Ann telling her that she must not wait for him. Larry then went out to fly a mission, during which he broke out of formation and crashed his plane, killing himself. Larry was reported missing.
Three years later, the action of the play begins. Chris has invited Ann to the Keller house because he intends to propose to her--they have renewed their contact in the last few years while she has been living in New York. They must be careful, however, since Mother insists that Larry is still alive somewhere. Her belief is reinforced by the fact that Larry's memorial tree blew down in a storm that morning, which she sees as a positive sign. Her superstition has also led her to ask the neighbor to make a horoscope for Larry in order to determine whether the day he disappeared was an astrologically favorable day. Everyone else has accepted that Larry is not coming home, and Chris and Keller argue that Mother should learn to forget her other son. Mother demands that Keller in particular should believe that Larry is alive, because if he is not, then their son's blood is on Keller's hands.
Ann's brother George arrives to stop the wedding. He had gone to visit Steve in jail to tell him that his daughter was getting married, and then he left newly convinced that his father was innocent. He accuses Keller, who disarms George by being friendly and confident. George is reassured until Mother accidentally says that Keller has not been sick in fifteen years. Keller tries to cover her slip of the tongue by adding the exception of his flu during the war, but it is now too late. George is again convinced of Keller's guilt, but Chris tells him to leave the house.
Chris's confidence in his father's innocence is shaken, however, and in a confrontation with his parents, he is told by Mother that he must believe that Larry is alive. If Larry is dead, Mother claims, then it means that Keller killed him by shipping out those defective parts. Chris shouts angrily at his father, accusing him of being inhuman and a murderer, and he wonders aloud what he must do in response to this unpleasant new information about his family history.
Chris is disillusioned and devastated, and he runs off to be angry at his father in privacy. Mother tells Keller that he ought to volunteer to go to jail--if Chris wants him to. She also talks to Ann and continues insisting that Larry is alive. Ann is forced to show Mother the letter that Larry wrote to her before he died, which was essentially a suicide note. The note basically confirms Mother's belief that if Larry is dead, then Keller is responsible--not because Larry's plane had the defective parts, but because Larry killed himself in response to the family responsibility and shame due to the defective parts.
Mother begs Ann not to show the letter to her husband and son, but Ann does not comply. Chris returns and says that he is not going to send his father to jail, because that would accomplish nothing and his family practicality has finally overcome his idealism. He also says that he is going to leave and that Ann will not be going with him, because he fears that she will forever wordlessly ask him to turn his father in to the authorities.
Keller enters, and Mother is unable to prevent Chris from reading Larry's letter aloud. Keller now finally understands that in the eyes of Larry and in a symbolic moral sense, all the dead pilots were his sons. He says that he is going into the house to get a jacket, and then he will drive to the jail and turn himself in. But a moment later, a gunshot is heard--Keller has killed himself.