The pitch :
A private investigator suddenly, mysteriously finds herself being thrown back in time on the first day of her menstrual cycle.
In this strange era she realizes that she's been presented with a mystery to solve. She learns that the only way to get back to her time period is to solve that mystery. She has until the end of her cycle to do so. If not, she's stuck until her next cycle begins the following month.
Every month she's thrown further back in time. The mysteries build upon each other, toward the solution to the mystery of why this started happening in the first place.