Don Gilet returns as Detective Inspector Mervin Wilson to confront a new set of mysteries on the beautiful island of Saint Marie. Still feeling like a fish out of water, DI Mervin Wilson continues to adjust to life on the island. The island agony aunt is poisoned by her own column, an academic is seemingly killed by a mythical sea monster, and a former hitman becomes the victim – of a hit. Meanwhile, a newly uncovered family connection brings more than Mervin bargained for.

Alphy’s really found a home in Grantchester. In Geordie too, he’s found a best friend and his intellectual equal. Love proves more elusive, until a case throws him in the path of a romance. But before he can let anyone else in, he must confront truths about himself. Geordie, meanwhile, wrestles with his expectations for his own son, as Cathy takes steps to better her career, aided by Mrs Chapman.