《摩斯探长前传》第四季于2017年1月回归ITV,本季共四集,标题分别为《游戏》、《圣歌》、《检疫》、《丰收》。剧情以1967年为背景,承接第三季结尾,由Shaun Evans和Roger Allam主演,Russell Lewis担任编剧,摩斯探长系列小说作者Colin Dexter担任顾问。Whilst Professor Amory unveils his chess-playing computer Jason and crime novelist Kent Finn signs copies of his latest book the body of Dr Nielsen, a member of the computer team, is dragged from the river and some time later a series of drownings are discovered at the local baths. The pattern of the murders suggests a serial killer with a knowledge of chess moves and Morse finds a link between one of the victims and another member of Amory's team with a further possible connection to Finn. However he deduces with some help from Jason that the culprit is not a random killer but one seeking vengeance on those he believed wronged him in the past. At the same time he feels unable to comfort Thursday and his wife after their daughter Joan mysteriously left them.