The sole survivor of a murderous ambush, a Belfast police detective is forced into a desperate search for a mysterious informer that takes him to a holy island on Lough Derg, a place shrouded in strange mists and hazy rain, where nothing is as it first appears to be. A keeper of secrets and a purveyor of lies, the detective finds himself surrounded by enemies disguised as pilgrims, and is drawn deeper into the mysteries of the purgatorial island, where he is forced to confront a series of disturbing secrets and ghosts in his own life.