After Nancy's crossbow shoots the Sin Eater, it turns back into Tristan. Nancy swears she will save him.

Ryan runs to Carson for help. Carson has them go to the Glasses, parents to parents, even though the Glasses hate Carson and, by association, Nancy.

Nick, Nancy, Bess, and George are trying to save Tristan. They call Ace, who tells them they need to pull the arrow out because more poison will spread as long as it's in Tristan's body. Nick pulls the arrow out while Bess does one of Temperance's spells to heal the wound. Then Nick cauterizes the wound, but they still need an antidote for the poison.

The poison is meant to kill the sin eater, so if it kills Tristan, it kills the sin eater. But since it didn't reach his heart, it could take up to 10 hours to work. Since the poison was made from the first sin, they need the last sin eaten by the sin eater to make the antidote.

Tristan's parents arrive with Ryan and Carson. The Glasses are furious. Tristan has no memory of anything that happens while he's the sin eater, so he wouldn't know the last sin eaten.

The Glasses got into the relic trade to look for an antidote for their adopted son. The Glasses adopted Tristan, but by age six, he started disappearing at night, getting sick, and having horrible nightmares. His soul is shackled to Horseshoe Bay, and he will die by age 30 if a cure isn't found. That's why they got into the relic trade.

They came close to a cure but mistranslated the symbols and accidentally made the eight bodies rise. Ryan offers to use his contacts to help them with the translation.

The Glasses know of a recall relic that could help them figure out the last sin, but when the antidote doesn't work, they realize they got it wrong, and there's another sin after the one they found.

Ryan and Shelby Glass search the Yacht Club for more answers. They find a camera in the fountain where the sinners wash their hands. Someone is watching and keeping track of the sinners. With only six hours left, Nancy uses Temperance's memory to weave dust. She and Nick go through Tristan's memories from two weeks ago. They see him consuming the sin and past sin eaters watching him.

Shelby and Ryan track the IP address connected to the camera. It leads them to a house on the edge of town. Shelby sees a printout of George at the fountain and someone else, but she hides it from Ryan.

After the memory potion, Tristan starts regressing through all of the past lives of the previous sin eaters. When they realize they got the last sin wrong, Nancy uses the memory potion again—this time, she finds Nashua, the original sin eater.

Nashua tells Nancy they have walked this path many times before. He takes her to the room where all of the sins are held, and they find the last sin and make the antidote. It's successful, and Tristan is saved, but neither he nor Nancy remembers the last sin.

At the morgue, Bess finds Ace sitting in a room with headphones on and his hands in a bowl of ice. Bess helps Ace talk to Jane Doe, the ghost. They decide that to help her, they must figure out where the real Mia is.

Bess and Ace search the woods. When they realize the trails have changed since the tsunami, they find Mia trapped between rocks with a broken let. As they save her, a large tree branch falls toward them. The ghost saves them.

Bess warns Ace about getting too attached to the spirit. It didn't work out well for her and Odette. The ghost shows herself to Ace after Bess leaves, and she is a beautiful young woman.

Happy to hear that Tristan is alive, Ryan tells Shelby that the mistranslated word was last sinner. Later, his contact sends him the full file. It says that the only way to break the sin-eater curse is to kill the last sinner's body.

With Tristan healed Nancy tells him she wants to pursue a relationship even though it will be complicated. Just as the two get close, Tristan's parents arrive and drug and kidnap them both.

It turns out that the other person who was photographed at the fountain was Nancy.