idée fixe

It was an idée fixe, you were a fanatic, you broke half a Crown Derby dinner-service when you couldn't get it.

But what they have placed in their heads, how can it be called other than 'a fixed idea' (idée fixe)?

They are the people of what the alienist calls the "idée fixe."

The story of this brave, patient little people, with its one idea—an idée fixe of liberty—began to move me.