On the darkest moon of Mars, scientists have found an ancient portal which allows teleportation throughout the universe. Considered to be mankind's greatest discovery, it's a gateway from hell, unleashing a swarm of demons looking to steal the souls of everyone who gets in their way. It's up to Lieutenant Joan Dark and an elite unit of Marines to destroy an ever-growing horde of bloodsucking and soul-stealing beasts.

It's 1957, and it's as if the world of Grantchester has returned to an idyllic state, a metaphorical Garden of Eden. Will believes that everyone can get close to their own Eden through good works and a proactive attitude. But as Geordie knows, every Eden has its snakes. Will's faith will be tested as he and Geordie are reminded that there's darkness lurking in their corner of Cambridgeshire.