The mystery of USA's limited series The Sinner has been why Cora Tannetti (Jessica Biel), a seemingly well-adjusted wife and mother, brutally murdered an apparent stranger without provocation. We've watched Detective Harry Ambrose (Bill Pullman) try to get Cora to remember whatever happened to her five years ago that triggered her violent reaction when she heard the music and saw Frankie Belmont (Eric Todd) making out with his girlfriend that fateful day at the beach. And in Wednesday's penultimate episode, "Part VII" we finally found out why.

Ambrose took Cora to the Beverwyck Club, a country club where something happened to her, to jog her memory. In "Part VI" she saw the stairs down to the basement and remembered, and at the very beginning of "Part VII" she and Ambrose went downstairs, which made the memories come rushing back. The rest of the episode after that was a flashback to the night Cora's sister Phoebe (Nadia Alexander) died.

Phoebe, of course, was Cora's younger sister, who had "Swiss cheese for a heart and a touch of non-Hodgkin's lymphoma" and had been sick her whole life and was prevented from having much of a life at all by their overprotective religious fanatic mother. Cora was her window to the world, and the girls experimented sexually so that Phoebe could know what those experiences were like. Their mother blamed all of Phoebe's misfortunes on Cora, a belief Cora had internalized as the events of the night crystalized.

Bill Pullman and Jessica Biel,<em> The Sinner</em>Bill Pullman and Jessica Biel, The Sinner

That night in 2012 was Phoebe's 19th birthday, and Phoebe persuaded Cora to take her with her when she snuck out to meet up with J.D. at the bar. J.D. offered them pills containing "molly and some other stuff," and Phoebe gulped one down, ignoring Cora's protests that it could kill her.

They went to the Beverwyck, and J.D.'s friend and bandmate Frankie Belmont was there waiting for them. He'd been squatting there while it was closed. Phoebe and Frankie hit it off. Frankie actually seemed like an alright guy; he was kind to Phoebe. They all went down to the basement to listen to J.D. and Frankie's music and do coke. They all began to make out with each other — Cora and Phoebe, then Cora and Frankie, then Cora and J.D. and all at once. That song from the beach started to play. Phoebe and Frankie were on the couch while the threesome was on the floor. Phoebe warned Frankie that she had a bad heart, and he kissed her scar and told her "there's nothing bad about this heart." But he was wrong.

Cora looked up and saw Frankie pounding on her sister's chest, trying to revive her. Her weakened heart couldn't handle it. There was a sickening crunch as Phoebe's sternum cracked. She was gone.

Not fully comprehending what was happening, Cora jumped up and tried to get Frankie off of her, executing the seven hits that Ambrose had noticed the music triggered. Then J.D. smashed her in the head with an ashtray, and the episode ended.

10 Ways The Sinner Is and Isn't Like The Cure's Song "Killing an Arab"

So we know why Cora killed Frankie: she was involuntarily reenacting the most traumatic moment of her life in some kind of regression. Which in turn means we know why Cora is the haunted shell of a person she is.

We still don't know exactly how or why that regression happened — what programmed her to react like that — or where she was for the months she was missing, or what happened to Maddie, or whose skeleton was found near the Beverwyck, or what's the truth about the guy in the creepy tan ski mask. Presumably all that and more will be answered in the finale next week.

But the show's original mystery has been solved.

The Sinner airs Wednesdays at 10/9c on USA.