One foggy night in Victorian London and a well-respected doctor is walking home after a night out, pausing to light his cigar, when a macabre, cloaked figure advances on him and strangles him to death. A passerby, journalist Bruce Adams (Craig Stevens), goes over to help but it is too late, and the next day he is out by a bandstand when he sees another man lying in the bushes - but he's only a tramp catching forty winks. Then Bruce is distracted by the suffragettes staging their protest for the vote, which consists of a song and a can-can, all of which degenerates into a riot...
Wait a minute, where are Bud Abbott and Lou Costello, as advertised in the title? Not to worry, for they're playing the policemen rushing to break up that fracas, and the explanation for how two Americans have wound up as London bobbies is in a throwaway line about teaching these imports the finer points of British police work. Not that this explains why most of the other denizens of the capital have American accents as well, but that can be solved when it was obvious this was Hollywood's version of 19th Century London, so some artistic licence was being employed. At least Boris Karloff's accent was English.
He played the other title character - well, sort of, as he was Dr. Jekyll, a scientist endeavouring to chemically separate the violent side of mankind from his peace-loving aspect to create a new era of "goodwill to all men" (Merry Christmas, Doc). Quite how this marries to his plans to eliminate his rivals and critics by necking the potion, turning into a monster then hunting them down and murdering them, is not entirely clear, as if you were analytical about this it would appear to be the antithesis of his hope for the future. Perhaps the experiments on his self have addled his mind, but the script was only really interested in producing a menace for the comedians to run away from, or indeed chase after.
Bud and Lou are absent for most of the opening twenty minutes where the plot has to be set up, with Jekyll's ward Vicky Edwards (Helen Westcott) being romanced by Bruce (Stevens would be recognisable to viewers of a certain age as the star of Peter Gunn on TV), a relationship much to the displeasure of Jekyll who lusts after Vicky himself. When Bruce looks like the potential next victim, our heroes foil the monster's plot by seeking to catch him and redeem themselves (they've been suspended for the disastrous handling of the riot), and though they only cause mayhem they do stop Jekyll in his tracks, leading him to invite them over to his place for security, but actually so he can get them where he wants them.
There was a question in the original Trivial Pursuit which enquired who played Mr Hyde in this film, and the answer it gave was Karloff, but that wasn't true, as stuntman Eddie Parker (also the Mummy in Abbott and Costello's next horror spoof) played him under the makeup, all the better for yet more running about. These setpieces, essentially getting chased one way then the other, may have served as a blueprint for umpteen episodes of Scooby-Doo but they grew monotonous in an admittedly short film, no matter that variations were conjured up such as a sequence in a wax museum where the boys could recall their biggest hit as models of Dracula and the Frankenstein Monster give Costello a scare. The biggest innovation came when Costello (his character name is Tubby, Abbott's is Slim - such imagination!) gets a dose of the potion and transforms into a giant mouse, then later becomes a fully fledged monster himself for mistaken identity farce. Not too bad, and it was a hit, but the invention was running out for the team.