Based on the novel by Madelon St. Denis, a murder committed during the shooting of a movie is solved by a publicity man in order to clear an actress.
A drive-by shooting is filmed on a set, but the actor Myles Brent is found to be dead. Studio owner Leon Grossmith (Alexander Carr) complains of the costs, and studio manager Joseph Steiner (Bela Lugosi) says it was an accident. The police find only blanks in the 45 guns, but Franklin Drew (David Manners) finds a 38 bullet and says it was murder. Police detective Sheehan (John Wray) questions actress Marcia Lane (Adrienne Ames) about her divorce from Brent and her refusal to sign a release on his life insurance policy. The electrician Chalmers (Alan Roscoe) is arrested, but Franklin shows his gun wasn't fired. They look at the film, but at the moment of the shooting the film catches on fire. A cigarette with Marcia's rouge on it is found in the projector. Someone also destroys the negative in acid. Franklin finds a gun in a light and is knocked out.
A studio guard (Vince Barnett) tells Franklin that Chalmers disliked Brent, because he got him fired. He and Franklin call on Chalmers and find him poisoned in apparent suicide; but Franklin notices a watch on his right hand with the pen by the note confessing to the murder and figures out he was murdered with battery fluid from Marcia's car. Leon offers to help Marcia, but she declines; he obviously resents her rejection. The guard lets Franklin into Brent's room, where he finds a letter from Agnes and a motel key. Sheehan questions Marcia in a studio office. Franklin promises to prove her innocent, but Sheehan arrests her. Franklin learns that Agnes Steiner sent flowers for Brent's funeral but tore up her check. At the motel he discovers that Brent had a fight with the woman's husband. From doodling he deduces it was Leon, Joseph Steiner, or director Tom Avery (Edward Van Sloan), all of whom have arranged to have Brent's double and Marcia shoot the scene again to finish the movie. The property man tells him he was ordered to use 38s; but as he is about to tell Sheehan by whom, the lights go out. Shots are fired, and Avery falls to his death. The studio guard catches the wrong man. In the final scene Franklin is congratulated by Sheehan and kisses Marcia.
This mystery is intriguing and has a surprise ending, since the audience expects Steiner (Bela Lugosi) or Leon to be the murderer. Publicity man Franklin turns out to be a master detective as he brilliantly solves the case in order to free Marcia.