Inside a beautiful Victorian family home in Glasgow's West End, a mother and her young son are found brutally murdered. DI Costello is furious and knows exactly who did it, George Haggerty, the husband and father. The only problem is that Haggerty has a cast-iron alibi - the police themselves caught him speeding on the A9 at the time of the murders. But Costello can't let it go. Determined to expose Haggerty as a ruthless killer, she's gone solo.

DCI Colin Anderson has no time to ponder his partner of twenty years going rogue, as his own cases are piling up. But Costello's absence becomes increasingly worrying. Has she completely disappeared following the tracks of a dangerous man?