At a bustling dance hall, Joe Loss and his Orchestra set the mood as Henrietta Jekyll, Maudie Edwards, leads her Old Time Dance Team. Her brother Henry Jekyll, Bernard Bresslaw, is a bungling, socially inept man who struggles to keep the routine on track. Their other brother Victor Jekyll, Jon Pertwee, steps in to conduct, but Henry’s awkwardness keeps threatening to derail the whole display. The crowd watches with a blend of amusement and concern as the formation falters and the performance hangs in the balance.
That same night, Henry heads back to the pharmacy where he works under the banner “Dr Henry Jekyll M.D. – Pharmacy, Estabd. 1812.” His elfin young friend Snouty, Jean Muir, appears in the rear yard and accuses him of being a square just as the lab suddenly explodes. In the rubble, a small metal box is revealed, containing a scroll with an old formula devised by Dr Jekyll in the 19th century. The writing promises a startling transformation: it can turn a man of timid disposition into a bold, fearless dragon. Henry, curious and hungry for confidence, drinks from the vial and is transformed into Teddy Hyde, a suave, self-assured figure who instantly turns heads at the dance hall and draws the attention of the local crowd and the ladies.
Teddy Hyde’s surprising charm is enough to catch the eye of the dance-hall boss, Dandy, Elwyn Brook-Jones, who recognizes a useful ally in this new persona. Hyde’s bravado impresses Dandy, who decides to recruit him for a robbery planned around the jewels on display. Before long, Henry awakens back in his bed with a pounding headache and a hazy memory, convinced it was all a dream. Yet fate intervenes again when Henry drinks the formula a second time, and Teddy Hyde reappears to join the heist with Dandy. The risks escalate as the night’s events unfold around a roof-light break-in and a line of thieves who intend to seize precious jewels.
Meanwhile, Snouty informs Victor that the mysterious Hyde was the source of the earlier disruption, and Victor realizes the family history may hold the key to stopping the trouble. Henry, however, remains blissfully unaware of the danger as he finds himself in possession of the jewels after the break-in—unwittingly leaving them in his pocket when the spell wears off and Dandy fails to recognize him in the daylight. The next morning, Henry’s memory remains muddled, but the jewels’ existence is undeniable, prompting Snouty to infiltrate Dandy’s office to learn their origin. She is eventually captured, and under pressure she reveals that Henry plans to return the jewels.
With Victor’s help, Henry begins to piece together his memories as Hyde and vows to set things right. The plan is to slip the jewels back to their rightful place during a forthcoming event that Henrietta and Reginald Bannister attend, while the police and show organizers keep watch. The pair face a high-stakes scramble on the stage and backstage, racing against time as the jewels go missing again and the crowd awaits the show to continue. On the night of the special event, the tension peaks as Dandy and a companion attempt another theft, but Snouty’s cunning and Henry’s Hyde persona prove a match for them. Victor and Henry's teamwork culminates in the boss’s defeat and the jewels being recovered just in time.
The story closes on a celebratory note as the show resumes with a flourish. In a memorable moment, Henry—still balancing his two selves—joins Snouty on the dance floor, with Snouty wearing a dress for the occasion. The two share a bold, unforgettable dance to cap the evening, symbolizing Henry’s acceptance of his Hyde-like bravado and Snouty’s unwavering support as the pair glide through the finale.
