A woman suspects her son is a Communist spy.

Dan and Lucille Jefferson, middle-class, religious Americans, live in a small town north of the nation's capital. They are disappointed when their son John, who works for the government in Washington, is unable to attend a Sunday dinner in honor of his brothers, Chuck and Ben, who are leaving that night for military service in Korea. However, the following Sunday, as Dr. Carver is delivering pills for Lucille's anxiety attacks and dizzy spells, John pays a surprise visit, his first in almost a year. Although John is greeted warmly by his father, a teacher, there is a distance between them as Dan staunchly espouses of traditional American values, while John wants change in the world. John and his parents then go to Mass, after which John is impertinent to the priest and, on their way home, asks to be let off to visit a professor for advice on a speech he has been invited to give to his alma mater's graduating class. While driving home, Dan tells Lucille that he feels that John has outgrown his family intellectually and has become condescending. Preoccupied by this thought, Dan is involved in a minor car accident, which is resolved amicably. When John returns home late that night, Dan tells him that he is running for commander of the local American Legion Post and John offers to read his speech, which is virulently anti-Communist. John mocks the speech and rewrites it. Later, Lucille also has difficulty talking with John and accuses him of deriding her. The next day, Stedman, the driver of the car Dan involved in the accident, comes to the house to explain that the repairs cost more than he had expected. Lucille welcomes him and tells him about her sons, including John. After Stedman reveals that he and John attended the same university, Lucille finally persuades him to tear up the repair bill. That night, Dan reviews the changes made to his speech and discovers that John has inserted ideological material with which his father does not agree. Dan tells his son that he sounds like a Communist and, if he were, he would take him out to the back yard and beat him. Dan then leaves for the Legion meeting to deliver his original speech, while John receives a phone call from a woman. After Lucille tells John about the accident and the other driver's visit, John makes arrangements to leave town early. Before he goes, however, he explains to his mother his belief that Americans must learn to help minorities and create a better-ordered world, but that his father regards this liberal philosophy as Communistic. John then swears on his mother's Bible that he has never been a member of the Communist party, and when Dan later returns from his meeting, he is pleased to learn that John has reassured his mother. However, Dan points out that if John were a Communist, his swearing on a stack of Bibles would be meaningless, and they become involved in an argument about the veracity of the Bible, which ends when Dan smacks John's forehead with the Bible and pushes him over, tearing his trousers. Lucille, greatly upset by the confrontation, sends Dan back to the Legion hall, while John changes his trousers, then leaves. Later, Dan returns intoxicated and angry that his son has mocked his beliefs and shows Lucille a newspaper headline about the sentencing of a female spy in Washington. The next day, John phones Lucille to ask for the damaged trousers, but learns that she has already taken them to the church clothing drive. Lucille, by now deeply worried by John's behavior, is about to return to the church to retrieve the trousers at her son's request, when Stedman comes to the house and reveals that he is an FBI agent. Stedman asks to talk with her about John, and although he states that he is not sure that John has done anything wrong, Lucille chooses not to answer his questions. After recovering the trousers, she discovers a key in a pocket. She then takes a plane to Washington, goes to John's office and gives him the trousers, but retains the key. When she tells John about Stedman's visit, John tries to convince her that Stedman was probably conducting a routine loyalty check, but Lucille senses that John is deceiving her. Later, while waiting for John near the Jefferson Memorial, Lucille is approached by Stedman, who has had her followed. Stedman appeals to Lucille's patriotism and asks her to accompany him to a jail to determine if the voice of the female spy is the same as that of the woman who phoned John. Wracked by doubts, Lucille agrees and, later, goes to the woman's apartment and opens it with John's key. Lucille, whom Stedman has had tailed and filmed, returns to her home, and Dan later finds her collapsed on her bed. As Dan leaves to fetch the doctor, John enters and Lucille tells him that she went to the convicted spy's apartment and knows that he was somehow involved with her. John then confesses that he and the woman had been having an affair and, although Lucille does not approve, she is relieved to learn that he has not been involved in anything treasonable. Suddenly, however, she becomes suspicious of John's story, and advises him to confess to the FBI. When Lucille refuses to hand over the key, John tries to convince her that she is ill and that no one will believe any report she may give. After Stedman arrives at the house and John informs him that his mother is delusional, Stedman discovers that Lucille is indeed delirious and accuses John of causing his mother's illness. When Dan and the doctor arrive, John slips out of the house as Stedman shows Dan his identification. Stedman then phones his office to report that the witness is ill and that her testimony may never be valid. Back inside, John overhears the doctor telling Dan that Stedman will have to let John go as he cannot prove him a traitor without his mother's testimony. Dan is distraught and enraged by his son's treachery, but joins Lucille in prayer for him. Later, from the airport in Washington, John phones Stedman to tell him that he has decided not to flee the country and wants to meet with him. At his office John finds a telegram advising him that his alma mater has decided to award him the honorary degree of Doctor of Laws. John, now deeply troubled by the conflicts with his parents and his mother's illness, revises his speech and makes a tape-recorded draft of it. Afterward, when John phones Stedman and asks him to listen to his new speech, Stedman realizes that the line is tapped and advises him to come to his office immediately. John takes a taxi, but is followed by another car. Shots are fired and the taxi crashes onto its side at the bottom of the Lincoln Memorial. The taxi driver is unhurt, but John is mortally wounded. John is still trapped inside by the time Stedman arrives, but with his dying breath, tells him about the recording. Stedman promises John that, if he thinks the students should hear the speech, they will hear it. At the graduation ceremony, John's recording is played verbatim. In it, John explains that he had intended to flee to Lisbon but realized that his conscience would not permit him to be free there. Somewhere along the way, he says, he substituted faith in man for faith in God. John's words warn the students that even now they are being observed for potential recruitment to the Soviet cause, and while exhorting them to hold fast to honor, confesses to being a traitor and Communist spy. John's speech ends with the observation that he expects to begin a new life with his arrest and asks God's help. As Dan and a recovering Lucille leave the ceremony to pray in a nearby chapel, Dan says that there was a lot of good in what John said. Lucille hopes that John's misdeeds will be forgotten, but that his words to the students will be remembered.