Has gangster Joe Gurney (Humphrey Bogart) the self-styled Napoleon of Crime met his Waterloo? Physician Carol Nelson (Kay Francis) suspected of having ties to the notorious killer must prove her innocence or the Medical Board will revoke her license. When Gurney seeks her out for treatment after being shot it could be the break Nelson needs. Now she has a chance to use her medical know-how to outwit Gurney and his goons and re-establish her professional reputation. King of the Underworld marks the lone movie pairing of two legendary Warner Bros. Stars. Francis a 1930s symbol of sophistication and couture was then on the downside of her career. Bogart whose name appeared above the title for the first time still was cast mainly in crime roles. But the films that would bring him prominence were on the horizon. The man many consider to be Hollywood's greatest-ever star would emerge from a celluloid underworld.