As Carl and his new assistant start digging into their first cold case, he can't stop his personal demons encroaching on his professional life.

Carl and Akram recreate the fateful ferry journey on their way to the Isle of Mhòr. A frustrated Rose makes her case for joining their Department.

Rose goes to the island to find out what Carl couldn't. Akram finds a note which sends the investigation in an entirely new direction.

The prosecutor's disappearance becomes connected with a death. A scuffle in a newspaper office sees Carl making headlines, and Moira sees a therapist.

The police detain a suspect for the Leith Park shooting. Carl returns to the scene of the crime, before losing his temper in public once again.

Carl makes some dangerous allegations to some powerful men, while Rose takes a fresh angle to uncover more information about Sam Haig.

The kidnappers give their victim a clue as they press her for an answer to their question. Dept. Q gets closer to unravelling its own riddle.

A decades-old recording helps all the clues to fall into place. But as the pressure rises — in and out of the chamber — can Carl get there in time?