In San Quentin prison, baby photographer and amateur detective Ronnie Jackson, awaiting execution for murder, tells the press the story of his demise: Ever aspiring to be a detective, Ronnie invents a keyhole camera lens and buys a gun, hoping to work for private detective Sam McCloud, whose office is across from Ronnie's studio in San Francisco's Chinatown. When Sam goes to Chicago, he leaves Ronnie behind to man the telephones, and Ronnie takes the case of Carlotta Montay, a beautiful brunette whose uncle, Baron Montay, is in trouble. Carlotta is being tailed by Kismet, the henchman of Major Simon Montague, who is trying to steal the mineral rights to Montay's uranium mine in San Dimas, California. Carlotta gives the encoded map to the mines to Ronnie, and he hides it in the cup dispenser for his water cooler. He then drives to an address Carlotta gave him on the Monterey peninsula, an unoccupied mansion being used as Montague's headquarters. Montague, believing Ronnie is Sam, tells him that Carlotta is suffering from delusional tendencies, but after Ronnie takes a keyhole photograph of the wheelchair-bound baron walking, he realizes that one of Montague's men is posing as the baron, and that Carlotta is in danger. Kismet follows Ronnie to Sam's office to confiscate the photograph and knocks him out, then burns the negative. Ronnie then brings two policemen to the mansion, but they find it deserted except for Kismet, who poses as an immigrant gardener. Kismet plants a clue in Carlotta's room, which leads Ronnie to the Seacliffe Lodge, an expensive sanitarium, where he is taken hostage and held with Carlotta and the real baron. While Montague's men are away searching for the map, Ronnie and Carlotta escape. Following a tip from the baron, Ronnie and Carlotta meet James Collins, the engineer who encoded the map, and Ronnie and Collins go to the police. Outside the police station, Kismet shoots Collins dead with Ronnie's gun. Wanted for murder, Ronnie, in disguise, travels to Washington, D.C. with Carlotta in order to expose Montague, who is to meet with a representative from the State Department named Dawson. Ronnie and Carlotta pose as hotel service staff, enter Montague's suite after his meeting with Dawson and record Kismet confessing to the murder of Collins. Kismet switches the record albums, however, and Ronnie is arrested instead. In the present, Ronnie is pardoned because Sam has discovered that one of Ronnie's clients had the negative of his keyhole photograph. Now, as Carlotta and Ronnie embrace, his executioner frowns, disappointed.