Gangsters force a crime writer to plan a murder for them in this adaptation of the play by Laurence Gross and Edward Childs Carpenter.
Jake Dillon (Edward Arnold) sends two men to pressure brewer Otto Barfuss (Joseph Cawthorn) into giving them $25,000; but they are arrested for extortion. Young Lombardo wounds a cop and is killed. Dillon assures Ricco Lombardo (C. Henry Gordon) his brother will be avenged.
The car of engaged couple Wallace Porter (Ernest Truex) and Toby Van Buren (Una Merkel) breaks down, and they enter Dillon's house to telephone. Dillon makes Porter take a drink and learns he writes about crime and could plan a "perfect murder." Dillon asks him to plan the murder of Barfuss and won't let them leave. Porter tries to telephone, but Dillon has Joe (Nat Pendleton) disconnect it. Toby wants to vamp Joe, but Porter won't let her. Porter offers Slim (Johnny Hines) $10,000 to help them get away and gives him a check; but Hilda (Marcelle Corday) tells Dillon about it. Newspapers are reporting Porter and Van Buren missing.
Porter explains how his plan works, showing how to put poison in a toothpaste tube on the train. He asks them why they want to kill a person and switches the poison with sugar. Dillon wants to try it on Slim as he finds Porter's check in his wallet. Joe indicates it worked, and Dillon thanks Hilda; but he still won't let Porter and Toby leave. The druggist informs Porter that Charlie (John Miljan) forgot the packet and is getting new poison. Porter drinks, and the lights go out. By candlelight he and Toby go to their bedroom. As this could be their last night, he gives her the wedding ring. She undresses and goes to bed, but he loves honor more than her. He prays for one good idea.
On the train Charlie puts poison in the toothpaste. Porter connects the phone line to the radio, figuring out how to send and receive messages from the operator. Joe comes in, but Porter and Toby in conversation give the information. Porter knocks Joe out with a poker. The police stop the train and page Barfuss. Porter tells Joe to get away. Cars leave Dillon with Porter and Toby. He pulls a gun, but Toby knocks it away. The police come in with her father, who now approves her marriage. Porter calls Barfuss and tells him not to brush his teeth.
This mystery thriller makes comedy out of the couple's predicament. Toby's morals are more flexible than Porter's, but they make a good team, outsmarting the unsophisticated gangsters, who are employed by the bootlegging Prohibition fostered.