Johnny Dangerously is a satiric take on popular gangster movies. It stars Michael Keaton and Danny DeVito. Johnny Kelly, played by Keaton, is newsboy working in New York City during the early 1910s. One day he fights with another kid and beats him up, attracting the attention of a crime lord. The crime lord, Dundee, offers Johnny money to do a job for him. Johnny desperately needs money because his mother is sick so he takes the man up on his offer.
Johnny helps the gangster break into a nightclub owned by a rival crime boss. Dundee is so impressed with Johnny's skills that he hires him full-time. Johnny keeps his chosen career secret from his family because he does not want them to be upset, but the rest of the neighborhood knows who he is.
Fighting persists between Dundee and Moronie, the rival crime lord. Moronie sends a robotic machine gun to attack Dundee's gang but it fails to kill anyone. Johnny gets Moronie back by dropping a bomb onto his nightclub, destroying it. At this point, Johnny has also started a romance with Lil Sheridan.
Tommy, Johnny's brother, becomes district attorney and is stunned to learn that his brother is a criminal. He begs him to try to quit his life of crime. The gang fears that Johnny has become an informant for Tommy so some of them plot to have him killed. Johnny is then arrested for a murder which he actually did not commit. The gang does not stand by him. Johnny's mother tries to launch an investigation, and she finds a woman who witnessed the crime. However, it is too late; there is a trial, and Johnny is sentenced to die. While in prison, he makes a gun out of spare parts and uses it to escape. Afterwards, he rescues Tommy, who the gang is trying to kill, and the Governor pardons him. Johnny then decides to give up his life of crime.