The Curious Case of the Suspicious Substitute

Severus has a problem. A completely illogical problem. Professor Regnarg is the new substitute professor, and Severus keeps finding her teaching all of the classes at Hogwarts. The problem is that Severus has the odd feeling that he's met her before, a feeling that only gets stronger the more he spends time with her. He must discover her true motives before it is too late.