An actor playing a notorious killer begins to live the part.

After performing the last act of the play The Brighton Strangler , London actor Reginald Parker puts his fiancée, playwright Dorothy Kent, on a train bound for Canterbury, promising to join her one week later when the play closes. On closing night, Reggie is in his dressing room when the Germans bomb the theater district, and he is hit on the head by falling debris. The dazed Reggie finds a luggage claim in his coat pocket and staggers to Victoria station, where he hears April Manby, a WAAF who is returning home for the holidays, purchase a ticket to Brighton. April's words are identical to a cue from the Brighton Strangler , causing the confused Reggie to believe that he is Edward Grey, the murderer he portrays in the play. Following his cues, Reggie buys a ticket to Brighton and introduces himself to April as Edward Grey. April is met at the station by her parents, who invite Reggie for Christmas Eve dinner the following evening. Meanwhile, in Canterbury, Dorothy reads the news about the theater bombing and, believing that Reggie has died in the attack, returns to London to identify his body. The next day, while staring at the sea from his hotel room window, Reggie, still believing that he is Grey, falls into a daze and becomes convinced he must kill the mayor, a crime that Grey committed in Act One of the play. That night, Reggie goes to the mayor's house and strangles him as he walks from the street to his door. After setting the dead man's watch ahead to eight o'clock, Reggie proceeds to dinner at the Manby house. There he is greeted by April, who confides that she is secretly married to American pilot Bob Carson. April asks Reggie to keep her secret from her parents, who have just lost their son, an RAF pilot. As the group sings Christmas carols, word comes of the mayor's murder. The next day, W. R. Allison, the chief inspector, sends his men to question everyone who has recently arrived in Brighton, but Reggie, who appeared at the Manby house at 7:45 the previous evening, has an airtight alibi. Later, when April learns that Bob is coming to Brighton on leave, Reggie offers to meet him at the station for her. After escorting Bob back to the hotel, Reggie retires to his room, where the sound of a vacuum cleaner triggers his cues from the play's second act in which he kills the Inspector General. Reggie begins to speak Grey's lines in his sleep, and Bob, overhearing his menacing tone, awakens him. Six days later Reggie, in an attempt to complete the second act, follows Allison to the cinema and takes a seat behind him. As he is about to strangle Allison, a newsreel of the bombing of London's theater district flashes across the screen, and Reggie, confused, stumbles out into the lobby. After the film ends, Allison sees Reggie seated in the lobby, and when Reggie, who is posing as a novelist, questions him about criminology, Allison agrees to meet him later that night to discuss the topic. Meanwhile, Bob has become suspicious of Reggie, but when he voices his misgivings to April, she accuses him of jealousy and reminds him that they need Reggie to help hide their marriage from her parents. In his role as escort, Reggie agrees to pretend to accompany April to a concert that night, but drops her off at the hotel to meet Bob instead. Leaving April with Bob, Reggie announces that he plans to attend the concert by himself. Soon after, Bob is called back to his base and April decides to join Reggie at the concert, but when she arrives at the auditorium, she finds his seat empty. As April listens to the music, Reggie visits Allison at his apartment. Aroused by Allison's collection of murder weapons, Reggie confesses to killing the mayor and then strangles Allison. His crime is interrupted by the entrance of Allison's neighbor, who speaks to Reggie, but whose blindness renders him incapable of witnessing the murder. After the concert ends, April returns to Reggie's hotel, and when she questions him, he becomes defensive. Reggie offers to walk April home, and as they stroll along the cliffs, he pulls a silk cord from his pocket and is about to strangle her when she mentions New Year's Eve. The words trigger a cue about the setting of the last act of the play, causing Reggie to feel faint. After April takes hold of his arm to steady him, Reggie tells her that he plans to return to London for New Year's Eve and suggests that she join him and see Bob. Meanwhile, in a London bar, Bob sees an advertisement featuring "Reginald Parker," and when he questions the waitress, she tells him that the actor played the Brighton Strangler on stage. Bob takes the poster to Inspector Graham, who states that Reggie was killed by a bomb. When Bob insists that Reggie is alive and masquerading as Edward Grey, they visit Dorothy. As they speak at her apartment, a news bulletin comes over the radio, announcing the strangulation of the Chief Inspector of Brighton. Dorothy, realizing that Reggie is acting out the role of Edward Grey, tells them that in the last act, Grey kills a woman at midnight on New Year's Eve in a deserted roof garden. Bob immediately calls Brighton and learns that Reggie and April have left for London. With twenty minutes remaining before the clock strikes midnight, Reggie accompanies April to a deserted roof garden of a hotel. Graham, Dorothy and Bob rush to one of the two hotels in London that have roof gardens, just as Reggie begins to speak Grey's lines and pulls the silk cord from his pocket. April is trying to flee her assailant when the others arrive on the rooftop. Seeing Reggie chasing April, Dorothy orders the others to applaud, and the sound of clapping causes Reggie to pause to take a bow. As he backs up, he falls from the roof to the ground. Before dying, Reggie opens his eyes, recognizes Dorothy and speaks the line, "the play has ended."