The Ugly Duckling (1959 film)

The Ugly Duckling is a 1959 British comedy film, directed by Lance Comfort for Hammer Film Productions and starring Bernard Bresslaw, Jon Pertwee and Reginald Beckwith. The film is a comic adaptation of the Dr. Jekyll and Mr. Hyde storyline and has nothing to do with the Hans Christian Andersen story.

The formula wears off, and Teddy changes back into Henry, who is appalled at the crimes committed by his alter ego. Feeling overcome with guilt, he turns coppers nark and helps the police to round up and capture the robbers who have evaded them for so long.

Source: Wikipedia