Cassie gets wired up, and to test the mic she recites an obscure poem called "At the War Cemetery, Bayeux" by Charles Causley. How literary.
Rob finagles his way into the van to follow along; they eavesdrop on the wire as Cassie visits Rosalind.
Rosalind agrees to go for a walk with Cassie, and, like Lex Luthor or the Joker, reveals her entire plan in a dramatic monologue.
In order to keep her going, Cassie pretends she's in love with Rob and can't tell anyone or she'll get fired. Rosalind feels superior because she knew this the whole time.
Keeping the maniacal laughter to minimum, she says she used to poison Katy, but when Katy found out, she had to find a way to kill her.
So she manipulated Damien so much that he thought he came up with the plan himself—you know, the plan to kill Katy in the woods.
Cassie arrests her, and they struggle. The guys from the van swoop in as backup and split them up, but not before Rosalind manages to claw her fingernails down Cassie's face.
Back at the station, Rosalind says, "Don't minors have the right to have a parent or guardian present during an interview?" (24.196)—turns out, the lying little… girl is seventeen. She simply told Rob she was eighteen. And he believed her.
And this means that everything she said on tape is inadmissible evidence. Way to go, Rob.