It seems to be an old one. It was on TV today, but in french so I understood very little. The main character is a girl who has blond hair tied back in a decent pony tail and a long sleeved blue dress and a cross. There is a convent and she talked to some nuns there. Her name is Simone.

Later, she changes her outfit and it turns out she's a crimefighting gal complete with a cape and her cap slightly askew. She wears an eye-mask like Zorro and fights with a fencing sword.

I can't find it for the life of me and I want to know! Any help?

La seine no hoshi :)

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