Frank (Tom Vaughan Lawlor) outlines his larger plan for Cassie (Sarah Greene) to go undercover as ‘Lexie’, to find her double’s murderer. Cassie (Sarah Greene) turns him down flat. Meanwhile, the Katy (Amy Macken) investigation grounds to a halt and Rob (Killian Scott) focuses his increasingly disturbed mind on tracking down Jonathan’s (Peter McDonald) two best friends from the past, who he is convinced hold the answers to both cases. As the pair are overwhelmed with memories of their traumatic childhoods, Rob and Cassie turn to one another for comfort and reassurance, but Rob can’t deal with any intimacy and he is absolutely savage in his rejection of her. Left reeling and feeling betrayed, Cassie agrees to Frank’s plan to go undercover and play the role of the murdered young woman. Dublin Murders - Tuesday at 9.00pm on BBC One.