In the sapping summer heat of 1665 there is little celebration in London of the naval victory at the Battle of Lowestoft. The King, his retinue and anyone with sufficient means has fled the plague-ridden city, its half-deserted streets echoing to the sound of bells tolling the mounting number of deaths. Those who remain clutch doubtful potions to ward off the relentless disease and dart nervously past shuttered buildings, watchful for the thieves who risk their lives to plunder what has been left behind.

At Chelsea, a rural backwater by the river, with fine mansions leased to minor members of the Court avoiding the capital, there are more immediate concerns: the government has commandeered the theological college to house Dutch prisoners of war and there are daily rumours that those sailors are on the brink of escaping. Moreover, a vicious strangler is stalking the neighbourhood.

Thomas Chaloner is sent to investigate the murder of the first victim, an inmate of a private sanatorium known as Gorges. There have been thefts there as well, but the few facts he gleans from inmates and staff are contradictory and elusive. He realises, though, that Gorges has stronger links to the prison than just proximity, and that the influx of strangers offers plenty of camouflage for a killer - a killer who has no compunction about turning on those determined to stop his murderous rampage.