Christian Maxwell is used to being alone. A delicate and over-imaginative young man, he's abandoned to his own devices and company in his family's Wiltshire country home. When his aunt, concerned for his well-being, insists he accompany her to the Welsh Borders for a country house party in the autumn of 1816, Christian is resigned to being surrounded by people for the duration.
One particular guest, Sam Gillespie, is a handsome amateur scientist of a similar age to Christian who is determined to befriend him. When Sam persuades Christian to join him on an expedition to the scenic ruins of Tintern Abbey, they stumble across an isolated valley that contains a long-dead and sinister monastic force.
Their dreams are haunted by the spirit of the place, so the pair band together to find out the valley's dark history, kindling their romance along the way. But their ghostly foe is more tenacious than they imagine. Can Christian and Sam manage to defeat the black monk and find lasting happiness?