You can't choose what will be your first task when you start working somewhere. It could be a complicated client, a very demanding chef, or a complicated case, like in Rachel's case.
The young Rachel started working as a detective. She might be an amateur, but that doesn't mean that she will be spared from difficult cases. She works on a case of a murder of a teenage girl, a case that is not that new but was never solved. The murderer of the girl was never found, so now he walks freely on the streets, like each one of us. That means that many people are in danger because it is not known if he has committed more murders and will he commit another one.
Rachel thinks that she should start the investigation from the beginning and try to find out some pieces of evidence that were missed out in the investigation. She goes to the place of the crime scene, in the house where the body of the girl was found at first, and starts the new investigation, Let's see if we can help somehow in the process of catching the murderer.