Episode 6
S4, E6: [Season Finale] The DNA on the scarf used as the murder weapon in Lizzie's murder matches Duncan's, and he is identified as Alan's father. At the same time, he is brought to the station as a suspect in the Lizzie and Sally cases.

S4, E6: [Season Finale] The DNA on the scarf used as the murder weapon in Lizzie's murder matches Duncan's, and he is identified as Alan's father. At the same time, he is brought to the station as a suspect in the Lizzie and Sally cases.