Episode 4 (E04)

The discovery of Lexie's body sends Cassie reeling back into her own past and gives us insight into why she chose the Garda in the first place. While the authorities search for who Lexie was, Frank and Cassie want to discover why she was murdered. Frank wants Cassie to go undercover in Whitethorn House, where Lexie lived with her four best friends. Cassie initially resists, but a disastrous night with Rob sends her running back to Lexie. (BBC)