Tom Fletcher may be an irresponsible ladies' man, but he is also the best fire investigator in the country. He is hired by the Interoceanic Fire Insurance Company to investigate a series of financially disastrous fires. Tom takes the job on the condition that he receive exorbitant perks and that his friend, John Grayson, be retained as the acting chief investigator. No sooner does Tom take the job than the Acme Fur company explodes into flames. As he investigates the scene, Tom meets Adrienne Martin, an attractive young woman who has been seen at numerous other fires. They are immediately attracted to one another, and John, also smitten by Adrienne, warns her that Tom is a Casanova. Tom discovers that all the fires were started by an arsonist disguised as a telephone repairman, and informs the insurance company of this. He then discovers that Adrienne has run a mysterious personal ad in the newspaper warning "F" not to use the phone again. At another fire, Tom discovers that the fire was set using a magnifying glass manufactured by the now-defunct Maxwell Company. Tom learns that the company went under when Interoceanic refused a loan to its owner, Erwin Maxwell, who swore revenge. Maxwell then committed suicide, though his body was never found. Tom searches Adrienne's apartment, and finds a passport in the name of Adeline Maxwell. He accuses her of being Maxwell's daughter, and has her confined to the mental ward of a hospital. Adrienne is sent a basket of fruit with the note "A--I got you into this and I'll get you out--F." Tom disguises himself as an orderly on the ward, where he witnesses a patient sneak into the paint room, which suddenly bursts into flames. Tom catches the arsonist, who turns out to be Erwin Maxwell. With the case solved, Tom fixes John up with Adrienne, who admits that she must be a little crazy, and tells Tom that he's a lucky man to have escaped her without being burned.