Since Rob is basically a non-entity now, Sam and Cassie try to discover Damien's motive for killing Katy, while Rob watches from the observation room.
They ask Damien about his involvement in the Move the Motorway campaign, and they reveal that they know all about Damien calling Rosalind from his cell phone.
Apparently, these two crazy kids met at a Move the Motorway protest and started a relationship.
Damien doesn't want her dad to know, though, because he would get mad at her.
Damien believes that Jonathan got mad at Rosalind a lot. Once he broke her skull with a poker, and he threw Jessica into a wall and broke her arm; Damien says Jonathan raped them, too.
According to Damien, Katy liked it. That's why she was Jonathan's favorite.
Rob knows this is all hooey, though, because they've seen the medical records.
Damien tried to get Rosalind to move out, but she didn't want to leave Jessica behind.
Damien suggested to Rosalind that he kill Jonathan, but she was afraid Katy would find another man who would do the same thing to them. Or worse.
So Damien decided to kill Katy instead. He considered it self-defense, protecting Rosalind and Jessica from her.
Rosalind lured Katy to the dig site by telling her that she had a friend who would give her a locket, which would bring her good luck in dance, and when she came to the finds shed to get the locket, he killed her.
Cassie wants to pursue Rosalind, whom she thinks is a clinical psychopath.
They don't need her parents, because she's eighteen, so Cassie decides to wear a wire and talk to Rosalind in hopes of getting her to confess.