Calder decides to stay on and investigate the previous night's discovery. Tosh welcomes her support as the revelation of a tattoo sends the case in a sinister new direction.When a mysterious tribute is left at the crime scene, Tosh and Calder are drawn to the house of Tom Knox, a nervous and reclusive figure whose family have a longstanding feud with the Bains. Tom is guarded about his relationship with Ellen and the detectives grow suspicious.Elsewhere, Sandy visits a mental health facility where Ellen stayed briefly before moving to London. He discovers that she was admitted by her cousin Heather and looked after by Dr Azir Sadat. Both blame Ellen's breakdown on her hard partying lifestyle but it becomes clear that there are hidden layers to Heather and Azir's relationship.Tosh receives a shocking call from Cora at the mortuary with a horrifying development, while Calder's past and present collide when she hears a startling revelation about her father.As Calder and Tosh race to prevent disaster, they are drawn to a horrifying fire on a remote hillside and a brand new prime suspect in this increasingly complex and unnerving case.