Lou drank one of the potions in Dr. Jekyll's collection, kept in his lab where he experimented with making potions that induced various transformations of people and animals. This potion caused Lou's head to transform into that of a giant mouse. This ability, whether induced by a potion or spell or occurring spontaneously, smacked of witchcraft or magic, thus Dr. Jekyll kept his experiments a secrete. This ability to create chimaeras of animals, including people, is a significant addition to the classic story by R.L. Stevenson. In contrast, in discovering the ability to change himself, physically and morally, into Hyde and vice versa, Jekyll had discovered the opposite: the transformation of a capacity for a mix of moral and amoral thoughts present, if sometimes latently, in every person into 2 distinct personas: one embodying a strictly moral person(Jekyll) and the other a strictly amoral person, such as Hyde, the latter being largely hidden from public view. This ability has a corollary in the real world in drug and alcohol-induced changes in personality, sometimes involving violence, and in multiple personality cases.
Rather like in "A&C meet Frankenstein", there are multiple monsters in this film, although most are of the same kind. Sometime after Costello spontaneously transforms from a mouse back into himself, he accidentally is injected with a serum that causes him to transform into a Hyde-like phase. This confuses people, as they think there is only one such monster who seems to be everywhere....In the classic tale, the Hyde phase of Jekyll eventually kills himself in Jekyll's lab, whereas in this film, he apparently accidentally falls from a window to the sidewalk. Like Costello, 4 police and an inspector turn into their versions of Hyde, after having been bitten by Costello in his Hyde-like phase. This reminds us of Dracula or the Wolfman's ability to transfer their persona to others by biting them. Hyde's head looked like a mix of Wolfman and a Chimpanzee, decidedly more monster-like than the impression from reading the original book. Also, his vocalization was like that of an aggressive monster. Batley: Dr. Jekyll's lab assistant was downright creepy looking and acting, reminding me of Frankenstein's monster, being tall, with a scowling scared face, and being silent.
At times, A&C exhibit their characteristic slapstick humor, and Lou's frequent mugging. However, there is none of their usual verbal humor. Much time is spent chasing or being chased by Hyde or Lou, especially within buildings or on rooftops. This probably appeals to most children, but not to me. My assessment of this film takes into account the probable reactions of children.
Boris Karloff actually only played Dr. Jekyll, even though he is credited with playing Hyde, as well. Stuntman Eddie Packer played Hyde, without being credited!...Two additional characters were introduced in the persons of Vicky Edwards(Hellen Westcott) and reporter Bruce Adams(Craig Stevens). The latter was sometimes involved in the chase for Hyde. Vicky and Adams were a developing romantic couple, as is found in most A&C films. Vicky was also the object of lust for Jekyll, who would occasionally turn into Hyde and carry her off.
Review by weezeralfalfa from the Internet Movie Database.