Season four picks up a year after season three, and business couldn't be better at the Wild/Reid Detective Agency as Harry and Fergus welcome an unexpected client: Charlie Wild. Elsewhere, Harry is hired by a friend and former wild child turned nun, Lola stumbles upon a dance school mystery, a Dublin literary tour takes a bloody turn and more. Meanwhile, Fergus' hopes of going to college at the same time as Lola are threatened when she gets an incredible offer to study abroad.

Harry and Fergus take on a disturbing caseβa serial killer targeting random victims and leaving a coin in their mouths. Meanwhile, Harry B asks a reluctant Harry to travel the world with him, and Fergus plans to attend college with Lola.

Harry and Fergus travel to the convent of a silent order of nuns after an old friend asks Harry to investigate a suspicious suicide, believing foul play is afoot. Elsewhere, a familiar face provides a very welcome surprise.

A fun day at the races for Harry and Fergus ends in tragedy when a racehorse throws its jockey from the saddle. Harry B discovers the horse was sabotaged and insists the owner, his father, Duncan, is responsible for the crime.

After two performers go missing from Genevieve Le Coeur's dance school, Orla and Lola persuade Harry to investigate. In search of the truth behind the disappearances, Harry and Fergus dive into the history of the school and its founder.

While attending a crime convention, Harry's look-alike is pushed to her death from a balcony. To stop the killer fleeing, Harry and Fergus get security to lock the building down while they investigate. A second murder reveals a chilling connection.

When a heckler dies during a guided tour, the investigation brings Harry and Fergus into the middle of a literature-themed murder plot. In America, FBI agents fail to prevent an assassination by Diyu, who plans their next hit at Vicky's wedding.