A dry but interesting take on the old tale, where the Hyde alter ego is the cleaned up charming version, albeit with a complete lack of morality. There's not much horror on display though, it plays out more like an infidelity fable.

This is a Hammer horror film that never feels like Hammer or horror. Hell, it barely feels like a Jekyll/Hyde story. In this variation, Doctor Jekyll is working on a formula to unlock a person's id, which just looks like a younger version of oneself, apparently.
This may as well be a mystery or film noir, since Hyde is far more interested in base-level hedonism than murder.
Did I mention Christopher Lee is in this? Well, he is, but playing against type for the horror icon that he was. Clearly this was pre-Dracula for Lee.
This isn't a bad movie, per se, it's just a weak retelling of a classic story about the evil that lurks within humans. Instead we have a story about Doctor Jekyll being a cuck.
The worst part is all the dance numbers. No, I am not joking.
I recommend this to no one.

Sadly, as most Hammer House horrors are, it was very slow building, and was more of a comedy than an actual horror.