A young boy disappears in San Diego on Halloween, just as another boy who vanished one year earlier suddenly reappears.
The team investigate when a psychology professor known for his controversial social experiments is found dead; Brennan works with Aubrey.
Emily and Aiden uncover long-buried family secrets and Victoria gets closer to exposing Emily, paying a surprise visit to Mason Treadwell. Conrad makes a declaration of war against Pascal's business.
When Ichabod gets Henry Parrish to help him communicate with Katrina, a threatening force is released from purgatory.
When Abbie and Ichabod learn Katrina is in great danger, they must race against the clock to help her.