If it seems like I've just spoiled you on the entire plot of the series, don't worry. The Sinner isn't a whoddunit mystery. We know exactly who killed Frankie Belmont (Eric Todd) and how she killed Frankie Belmont and where she killed Frankie Belmont. What we don't know is why Cora suddenly seized the knife she was using to cut fruit for her son and went to town on a guy at the beach. That's where Detective Harry Ambrose (Bill Pullman) comes in. Convinced that there's more to the story than that Cora simply snapped out of nowhere, he begins to dig into her past, and he works to uncover secrets about Cora's family that might help make sense of the vicious crime of which Cora is quite undeniably guilty.