A rash of horrific murders has terrorized London and baffled police. Newspaper reporter Bruce Adams finds one murder victim, a prominent doctor, while returning home from a pub. The next day, two American policemen, Slim and Tubby, who are studying London police methods, respond to brawl at a women's suffrage rally in Hyde Park. Reporter Adams, young suffragette Vicky Edwards, Slim, and Tubby are all caught up in the fray and wind up in jail. Vicky's guardian, Dr. Henry Jekyll, bails Vicky and Adams out, while Tubby and Slim are kicked off the police force. Unknown to anyone, however, Dr. Jekyll has developed an injectable serum which transforms him into Mr. Hyde -- the "monster" responsible for the recent murders. Dr. Jekyll, who is secretly in love with Vicky, notices Vicky's and Bruce's mutual attraction. He injects himself to transform once again into Hyde with the intent of murdering Adams.

Meanwhile, Tubby and Slim realize that they must capture the monster in order to be reinstated on the police force. Walking down a street at night, Tubby spots Hyde (whom Slim at first mistakes for a burglar) and the boys trail him into a music hall where Vicky is performing and Adams is visiting. A chase ensues, and Tubby manages to trap Hyde inside a cell in a wax museum. However, by the time Tubby brings the police inspector, Adams and Slim to the scene, the monster has reverted to the respected Dr. Jekyll, and Tubby is once again rebuked by the police inspector. The "good" doctor, however, asks Slim and Tubby to escort him to his home. Once at Jekyll's home, Tubby and Slim snoop around and Tubby drinks a potion which transforms him into a large mouse. Slim and Tubby bring this news to the inspector, but the inspector refuses to believe them.

Later, Vicky announces to Jekyll her intent to marry Adams, but Jekyll does not share her enthusiasm and transforms into Hyde right in front of her and attacks her. Adams, Slim and Tubby save her in the nick of time, but Hyde escapes. During the struggle, Jekyll's serum needle falls into a couch cushion, which Tubby accidentally falls onto, transforming him into a Hyde-like monster. Another madcap chase ensues, this time with Adams chasing Jekyll's monster and Slim pursuing Tubby's monster, since both believe they are after Jekyll. The police are frustrated and confused by reports of the monster seemingly being in multiple places at once.

Adams' chase ends up back at Jekyll's home, where Hyde falls from an upstairs window to his death, then transforms to his true identity. Meanwhile, Slim brings Tubby, who is still in monster form, to the inspector's office. Tubby bites the inspector and four officers, then reverts to his true self. Before Slim and Tubby can be reprimanded, the inspector and the officers transform into monsters. Slim and Tubby dash through a wall and out of police headquarters.