After the die-hard romantic paramour of a married woman is asked to help kill her abusive stepfather, he plans the perfect murder but when he discovers she is using him as a pawn, heartbroken, he plots his revenge.

The twist should not be about "Plotting"

Instead, it should happen quickly, and as a big reveal.

The very trap that was supposed to "get" the husband is expertly turned around on the vengeful wife when it's revealed Her Paramour and Her husband are lovers.

(Or the other way around. The husband convincing his lover to kill his wife only to be double-crossed by the lover and his wife at the end)

What's the story hook?

The story hook in "Les Diaboliques" is that the conspirators execute the perfect murder plan -- and then the corpse disappears.? And the corpse needs to be discovered for their plan to work.

What's the story hook?

The story hook in "Les Diaboliques" is that the conspirators execute the perfect murder plan -- and then the corpse disappears.? And the corpse needs to be discovered for their plan to work.