The Quality of Mercy opens in the spring of 1767, in the immediate aftermath of the events in Barry Unsworth's Sacred Hunger. It follows the fortunes of two central characters from that book: Sullivan, an Irish fiddler, and Erasmus Kemp, the son of a disgraced Liverpool slave-ship owner who hanged himself.
To avenge his father's death, Erasmus Kemp has had the rebellious sailors of his father's ship, including Sullivan, brought back to London to stand trial on charges of mutiny and piracy. But as the novel opens, a blithe Sullivan has escaped and is making his way on foot to the north of England, stealing and scamming as he goes.
His destination is the colliery village where his dead shipmate, Billy Blair, lived: he has pledged to tell the family how Billy met his end.
In this village, Thorpe in the East Durham coalfields, live Billy's sister Nan and her miner husband, James Bordon. Their three sons are all destined to follow their father down the pit. The youngest, only 7, is enjoying his last summer above ground. The terrible conditions in which mineworkers laboured are vividly evoked, and Bordon has dreams of escaping the mine with his family.
Meanwhile in London a passionate anti-slavery campaigner, Frederick Ashton, gets involved in a second case relating to the lost ship. Erasmus Kemp is claiming financial compensation for the cargo of sick slaves who were thrown overboard to drown, and Ashton is representing the insurers who dispute his claim. Ashton triumphs in court, but not before his beautiful sister, Jane, has encountered Erasmus Kemp and found herself powerfully attracted to him despite their polarised views on slavery.
She discovers that Kemp wants to spend some of his sugar and slavery fortune on Britain's new industries: coal-mining and steel. A landowner father of a friend of Jane's tips him off about Lord Spenton's mines, for sale in East Durham, and Kemp sees the business opportunity he has been waiting for.
Thus he too makes his way north, to the very same village that Sullivan is heading for . . .