A distraught woman arrives at the Eastvale police station desperate to speak to Detective Chief Inspector Alan Banks. But since Banks is away on holiday, his partner, Annie Cabbot, steps in. The woman tells Annie that she's found a loaded gun hidden in the bedroom of her daughter, Erin--a punishable offense under English law. When an armed response team breaks into the house to retrieve the weapon, the seemingly straightforward procedure quickly spirals out of control. Thrust into a complicated and dangerous case intertwining the personal and the professional as never before, Annie and Banks must risk everything to outsmart a smooth and devious psychopath.