Summary:

Mia's never been much of a worrier, but when her best friend Sam starts disappearing every night and losing her temper of the most menial of things, she can't help but feel a little out of sorts about the whole thing.

Mia and Her friend Ellie decide to investigate the situation, stealing Sam's phone and finding a few texts from a mysterious contact known as "Ronan". The texts describe a strange ritual called "shading", and discusses, chillingly, "sacrifices". Further investigation leads them to a mansion in a neighboring town, which is on the market to be sold but seems to have been taken over by the shaders. Mia and Ellie find themselves in the middle of a shading ritual, powerless to move as Ronan in the others burn something which seems suspiciously like a human heart. After this, everyone, including Mia and Ellie, are immersed in a euphoric hallucination. Though Mia and Ellie at first suspect that they had taken some sort of drug such as LSD, further research proves that this is not feasible. 

Mia and Ellie sneak into the mansion for a second time, where in the attic they uncover many astoundingly beautiful pieces of artwork, including some by Sam, who previously had not had a single ounce of artistic talent. Mia hypothesizes that shading may be some sort of way to become more artistically productive, but Ellie is skeptical.

After exiting the mansion, Mia decides that she wants to swim in it's pool and persuades Ellie to let her do so. Everything seems fine until Mia uncovers a body at the bottom of the pool, which is bleeding in the water and appears to be missing it's heart. Deeply rattled, she tells Ellie about this, but Ellie believes that she is making it up. Another body is found in the woods by police the next week, but Mia does not think it is the same one she saw in the pool because of a difference in hair color. Sam also offers a false explanation for her strange behavior, telling Mia and her friends that she is going to Harvard next year.

Mia leaves to visit her father in New York city for the summer, where she is annoyed that she cannot make any progress on the case because she is so far away. In a strange coincidence, Mia meets Scarlett, a girl she had observed partaking in the shading ritual back at home in Vermont. Scarlett does not know that Mia has previously encountered her, and tells Mia that she is no longer part of the shaders (though she does not mention by name). She also lets slip that Sam has gone to Canada with Ronan. 

Mia returns home, where another child has gone missing and Sam has supposedly gone to Harvard. Mia meets with Ellie to discuss the case, but finds that Ellie is acting strange and distant, and perhaps withholding information from her. Ellie then pushes Mia into a pool, which is incredibly traumatic for her because of the body she found in the pool behind the mansion. In a fit of rage, Mia steals Ellie's phone but does not find any texts on it that might give her a clue as to what the cause for Ellie's strange behavior is. Mia makes plans to meet with Scarlett for coffee, hoping to get more information out of her.