A new body is brought into the morgue. They believe it is the body of the missing camper, Mia Preston, but the body is charred beyond recognition.

The thermostat at the morgue is broken, and it's freezing. Then the sink turns on by itself. The radio turns on and off. The lights swing from the ceiling.

Ace asks Mia if she did that, and one light flickers. As Mia tries to communicate with Ace, he realizes that the body isn't Mia.

Every day since the ghost web, Tristan has been picking out the nicest shells from his catch and leaving them at Nancy's door with a message asking her out. She finally says yes, and plans to see him after they kill the sin eater.

They have to figure out what was the first sin the sin eater ate by returning to the black door mirror. The sin eater is vulnerable under a black moon. They have 48 hours. Nick built an extractor to get the mirror to show them the first thing it's ever seen.

It turns out the founders of Horseshoe Bay created the sin eater, and Addy's parents know the story from their ancestors.

In the early 1800s, the governance landed in Horseshoe Bay. They called themselves settlers, but really they were newcomers by about 12,000 years. At first, it was peaceful between the native tribe and the town founders. But then, one morning, the settlers invaded and kidnapped every child old enough to walk. The founders refused to return them. When the indigenous people tried to rescue their children, the settlers decimated them. The kidnapped children were never heard from again, and the founders' children inherited the land. The few ancestors who escaped kept an oral history. This was the first sin that the sin eater erased.

It's Bess' day in court. The prosecutor drops the breaking and entering charges but then charges Bess with one count of violating community standards, one count of endangering children, and one count of disseminating false information to the public. They offer her a plea deal. If Bess locks up all the books in the Historical Society so they are no longer accessible to the public, submits all artifacts to be kept in a court-mandated high-security facility, and makes a statement that it's wrong to teach about paranormal phenomena and admits they have never been a benefit to the community, the ADA will drop all charges.

Bess refuses the plea deal and testifies that the supernatural is no threat and can heal and even save lives if used correctly. Then she uses a potion to show the ghost mural that was painted over. The ghost mural shows the founders kidnapping the indigenous children and accuses the town of having a history of erasing history to make itself look like the good guys.

The judge finds Bess not guilty but accuses her of mismanaging the Historical Society. The judge turns over the Historical Society to Carly of the Family For Truth in History.

Bess figures out how to create a poison to kill the sin eater. With Ace's help, George volunteers to sin. She breaks Conner's beloved mug at the morgue and then goes to the black mirror and summons it to erase her sin.

When the sin eater is called, Nancy and Ryan head out with a crossbow and the poison Bess created to kill the sin eater. Nick and Bess stay behind to figure out what happened to the missing children from the first sin.

Nick finds hidden pages in one of the books Nancy took from the Yacht Club. The children were stolen to be used as guinea pigs because in order for the sin eater to survive, its soul must stay intact in a human.

Nancy believes she's going to kill a monster, but Nick calls to stop her because the sin eater lives in a human being. Unfortunately, Nancy shoots the sin eater with the poison arrow, and it collapses. That's when she sees the shell in its hand. Nancy is in shock when she recognizes the shell she sent back to Tristan, saying she would meet him for dinner tonight.