Jim “Brad” Bradley, an American trumpet soloist, is appearing with a locally engaged band at the London Palladium, Britain’s foremost vaudeville theater. On the drive back to his hotel, Brad hears a female singer’s voice coming from a closed nightclub and decides to join in a jam session with the singer, Maxine Halbard, and her musicians. Afterward, Maxine invites Brad to her apartment for a meal, and after a pleasant evening, he invites her out to dinner the following night. Maxine agrees, but warns him that she has a Canadian boy friend she is going to marry. As Brad departs, he absentmindedly leaves his trumpet behind and a shadowy figure approaches Maxine’s door. Early the next morning, Brad is awakened by police officers MacKenzie and Mulrooney who inform him that Maxine has been murdered and that he is a prime suspect. In addition to Brad’s trumpet, the police have also found a privately recorded phonograph record of Maxine singing, accompanied by pianist Jeff Colt, who is also a suspect. Later, Colt phones Brad and tells him that he is sending him a copy of the record. Meanwhile, Brad locates Maxine’s sister, Barbara Quigley, who is also a singer, at a rough club in Soho. Barbara is in love with her pianist Johnny Sutherland, but he was romantically involved with Maxine. Later, Brad listens to Colt’s record and invites him to his hotel for a drink. When Brad asks Colt to reproduce a passage he played on the record, he cannot. Colt then asks Brad to tell this to the police as he claims he never worked with Maxine and that another pianist was playing on the record. Although Brad’s manager, Max Margulis, informs him that he has lined up several engagements and recording sessions for him, Brad misses them to search for Johnny, who also has a copy of the disc. Johnny explains to Brad that the record was produced by Maurie Green, the owner of a company specializing in audition discs. Brad then goes to see Green, pretending to be interested in recording some practice discs. When Brad casually asks if Maxine and Colt ever recorded in his studio, Green says no. Later, as Barbara prepares coffee at her apartment, Brad looks through her photograph album and finds a photo of Barbara, Maxine and Colt’s wife Gloria billed as a singing “sisters” act. Barbara then explains that after the act broke up, she appeared with Johnny as his target in a trick-shot circus act. Realizing that Maxine took Johnny away from Barbara, Brad begins to suspect that Barbara may have killed her sister. Brad then gets Johnny to admit that he made the recording, imitating Colt’s style. Later, Brad saves Barbara from an intruder who has entered the apartment to remove a photo from her album. After further investigations, including a visit to a mental hospital, Brad phones police inspector MacKenzie to say that he knows who killed Maxine. The inspector orders all involved in the case to assemble at Brad’s dressing room at the Palladium. While rehearsing, Brad passes out and, when he recovers, a doctor informs him that the trumpet’s mouthpiece had been poisoned. By this time, all the principals in the case, as well as the police, have arrived and Brad reviews the events of the night of the murder. During his investigations, Brad discovered a photograph of Maxine and Green on the grounds of a mental hospital. Upon visiting the hospital, Brad discovered that Green had suffered a mental breakdown and that his medical file described him as a dangerous paranoiac. Brad continues that after Maxine had rejected Green’s romantic advances, he was pushed over the edge. When Brad accuses Green of the murder, Green pulls out a gun and attempts to shoot Brad, but Max jumps in front of Brad and is shot in the arm. The police then overpower Green and take him away. Later, Brad resumes his performances at the Palladium as Barbara and Johnny, now romantically involved, watch from the wings.