The librarians come upon a terrified young girl running from an old house. Jacob tells her she won't have to go back in again.
Cassandra works on figuring out what is happening, and her tumor starts to bother her.
The Librarians enter the house to check things out. The house begins to shake, and a knife flies into the wall. The run away as they watch the house.
Cassandra feels as though she is being sidelined. Katie runs back into the house to find her friends. They run in after her, and they hear someone walking above them. Jacob realizes the dimensions of the house are wrong.
They explore the house and find some strange things. They all happen upon the same picture but in different rooms. Eve sees kids screaming, and brings back their clothes.
Cassandra is still upset at being sidelined, thinking she can help in other ways.
Eve calls Jenkins to ask about ghosts in the house. Jenkins says they are in the "shadowbox" and they are the first librarians to set foot in it. He's says that once the house takes victims, there is nothing else they can do.
They have to find the dark heart of the house before midnight.
Music plays, and Katie's name is burned into the wall. Eve sends Cassandra outside and keeps Jacob there to fight.
Eve tells Cassandra to take Katie to the annex, but the shadow in the house attacks the car. It turns out the monster is the dark heart of the house.
Jacob finds Ezekiel in a dollhouse.
Something is off with Katie, and she seems to be evil. She sings children's songs as she heads back into the house.
Cassandra finds herself facing off against Katie. Katie says the house only talks to her. Cassandra asks the house to talk to her too.
Cassandra stabs Katie, and the dark shadow appears. Katie is the dark heart. Cassandra says she sees death every time she looks in the mirror, and that gives her the power here.
Cassandra cuts her and she turns to dust.
Cassandra finds the others and explains that the house was meant as a refuge before the family of serial killers found it.
As they drive off, we see that Ezekiel stole the doll house.