Andy wakes up and finds Nikki wading into the lake when she goes under. He tries to find her and can't. It's sort of like a flashback. The cops arrive on scene and search the lake looking for her.
The old guy from the boat is there to comfort Andy. The police find Nikki and Andy runs to the body. She drops a rock from her hand and he goes to pick it up.
He wakes up for real and finds he's tied to the bed with Marcus and Tomas performing an exorcism on him. He starts talking about Nikki and they tell him she's just a thing.
He has another flashback where Nikki talks about bringing another girl into the family. It was Verity. Then he's at a memorial in front of the house with the kids spreading Nikki's ashes.
Marcus and Tomas keep trying to get Andy to tell them how the evil first came to him. Andy goes back into his memories. Verity and Andy talk about Nikki killing herself.
He's up in the painting room when Grace shows up -- the first time the demon came into his life. Grace disappears and Nikki shows up. She tells him that they are trying to break them apart and asks him if he's going to let that happen and he says no.
The demon takes Andy on a trip to the park. Then it takes him to a den she made. She's trying to make him forget about the kids. She takes him to her studio. She converted Verity's room. She keeps telling him the kids are in the garden. She buried them there. But when he digs, the kids aren't there.
Nikki tries to justify why she got rid of the kids. He realizes that she's not his wife. He tries to break free from her, but then she wants to show him memories of what the kids were like. She claims Nikki killed herself because of the kids.
She shows him more memories. But he hears Marcus and Tomas talking to him to fight back.
The demon tricks Andy into thinking Tomas and Marcus are working against him and asks her for help.
And goes back into the lake and he choose Nikki over his kids, saving her. The demon has integrated into him. When Marcus and Tomas thought they were ahead of the game, they weren't.