《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。

Outside Parliament in Reykjavik, a man throws himself at the Minister for Industry, setting fire to them both. He dies, but she survives. Andri Olafsson, now working in Reykjavik, is assigned to the case.

the police duo Andri (Ólafur Darri Ólafsson) and Hinrika (Ilmur Kristjánsdóttir), get drawn into a war in the remote highlands, where two rival groups fight over specific pieces of land, but for very different reasons. The neopagan and peaceful sect, The Extended Family, led by its founder and spiritual leader Oddur (Egill Ólafsson) has raised camp on the land, see the land as sacred and want to be close to their heathen Gods. A group of Icelandic bikers, led by an angry young man, Gunnar (Haraldur Stefánsson), also claim the "sacred site". In order to take ownership of the land, he calls for backup and soon a group of Danish bikers arrive by ferry, led by the Danish leader, Hopper (Thomas Bo Larsen). The clash of the two groups leads to the death of a young man, Ivar (Auðunn Lúthersson). It turns out that Ivar was not unknown to Andri, so he feels obliged to join Hinrika in the North to investigate the murder case.

In an attempt to fix his financial problems, Erik recruits his brother, Atli, to help him import a shipment of cocaine into Iceland. Erik thinks he's got things all figured out, until the young Polish “mule” they've hired is unable to pass the drugs through her system.