Still traumatised by her brush with a psychopath, Detective Cassie Maddox transfers out of the murder squad and starts a relationship with fellow detective Sam O'Neill. When he calls her to the scene of his new case, she is shocked to find that the murdered girl is her double. What's more, her ID shows she is Lexie Madison - the identity Cassie used years ago as an undercover detective.

In Broken Harbour, a ghost estate outside Dublin - half-built, half-inhabited, half-abandoned - two children and their father are dead. The mother is on her way to intensive care. Scorcher Kennedy is given the case because he is the Murder squad's star detective. At first he and his rookie partner, Richie, think this is a simple one: Pat Spain was a casualty of the recession, so he killed his children, tried to kill his wife Jenny, and finished off with himself.

The photo shows a boy who was murdered a year ago.The caption says, "I know who killed him". Detective Stephen Moran hasn't seen Holly Mackey since she was a nine-year-old witness to the events of Faithful Place. Now she's 16 and she's shown up outside his squad room, with a photograph and a story. Even in her exclusive boarding school, in the graceful golden world that Stephen has always longed for, bad things happen and people have secrets.

The course of Frank Mackey's life was set by one defining moment when he was 19. The moment his girlfriend, Rosie Daly, failed to turn up for their rendezvous in Faithful Place, failed to run away with him to London as they had planned. Frank never heard from her again. Twenty years on, Frank is still in Dublin, working as an undercover cop. He's cut all ties with his dysfunctional family. Until his sister calls to say that Rosie's suitcase has been found....

Cal Hooper thought a fixer-upper in a bucolic Irish village would be the perfect escape. After 25 years in the Chicago police force and a bruising divorce, he just wants to build a new life in a pretty spot with a good pub where nothing much happens. But when a local kid whose brother has gone missing arm-twists him into investigating, Cal uncovers layers of darkness beneath his picturesque retreat, and starts to realize that even small towns shelter dangerous secrets.

Antoinette Conway, the tough, abrasive detective from The Secret Place, is still on the Murder Squad, but only just. She's partnered up with Stephen Moran now, and that's going well - but the rest of her working life isn't. Antoinette doesn't play well with others, and there's a vicious running campaign in the squad to get rid of her.

In the midst of a rock festival, a charity worker is sliced across the stomach. He dies minutes later. In a crowd of thousands, no one saw his attacker. The following week, the body of a primary school teacher is found in a Dumpster in an Edinburgh alley, strangled with her own woolen scarf. DI Ava Turner and DI Luc Callanach have no motive and no leads - until around the city, graffitied on buildings, words appear describing each victim.

As funeral mourners stand in silence at Ragmullin cemetery, a deafening cry cuts through the air. Lying crumpled at the bottom of an open grave is the bloodied body of a young woman, and Detective Lottie Parker is called in to investigate. Knowing the body can’t have been there long, Lottie wonders if it could be Elizabeth Bryne, a young woman who vanished without trace just days earlier. And with a new boss who seems to have it in for her, Lottie is under pressure to solve both cases quickly.

One hot summer evening, 11-year-old Mikey Driscoll is on the way home from playing with friends. Two days later, his body is discovered on a bed of wildflowers by some local teenagers. The case is assigned to Detective Lottie Parker, and this time it’s personal. The victim was a close friend of her son, Sean, from the run-down Munbally estate on the other side of town. Sean tells his mother Mikey was behaving normally before he died, but Lottie can’t help but feel that her son is keeping something from her.

When psychological profiler Tony Hill comes across a series of suicides among women tormented by vicious online predators, he begins to wonder if there is more to these tragedies than meets the eye. Could it be murder? But what kind of serial killer wants his crimes to stay hidden? Tony and DCI Carol Jordan begin the hunt for the most dangerous and terrifying kind of killer - someone who has nothing to fear and nothing to lose.